When comparing Mozilla Firefox vs Opera, the Slant community recommends Mozilla Firefox for most people. In the question“What are the best desktop web browsers?” Mozilla Firefox is ranked 1st while Opera is ranked 6th. The most important reason people chose Mozilla Firefox is:
Firefox [scores strongly on HTML5 feature support](http://html5test.com/results/desktop.html).
Ranked in these QuestionsQuestion Ranking
Pro Strong HTML5 feature support
Pro Free, open source and community driven
Firefox is available as a free download. All Mozilla software is licensed under the Mozilla Public License. Instructions on how to obtain the source code can be found here.
Pro Syncs between devices
Firefox Sync is an optional feature in Firefox that allows syncing bookmarks, passwords, and add-ons between devices.
Pro Strong developer tool
The built-in developer tools have been merged with the popular FireBug extension since FF57
Pro Awesome customizability
Pro Automatically updated
Firefox is automatically updated on the platforms where it makes sense.
Pro Reader View
Reader View in Firefox allows users to read an article without any distractions by removing ads, unrelated elements and other distractive objects (similar to Microsoft Edge's Reading Mode and Safari's reader mode).
Pro Good font rasterizing
Font rasterizing on Windows is much better than in competitors. Even smaller text is clear and contrast.
Pro Fast bookmark management
In order to add an open page to the bookmark bar, the tab can be dragged down and is added immediately.
Pro Ethical and pragmatic company mission
The Mozilla Manifesto outlines the company's mission and principles. Paraphrasing, they want the Internet to be a free and open resource, and to enable individuals to get the best use of that resource. They do this by creating open source software to which anyone may contribute, so long as such contributions fit with their principles (both ethical and technical).
Pro Tagging bookmarks
Firefox is one of the few browsers that you can tag your bookmarks. You can view a list of tags and can search your bookmarks in the address bar with tags.
Pro Use Less Resource
Firefox 57 (Quantum) and newer uses less resources than ever. It is proven with benchmark done by AppleInsider.
Pro High performance
The Firefox Quantum update (FF57) greatly increases the render speed and general performance of the browser, by taking better advantage of the user's hardware.
Pro Lower memory fingerprint than competitors
Firefox used to be a trailer in memory usage, but as of 2017 it's less hungry for memory than competitors like Edge, Chrome, Safari and Opera.
Pro Integration with Pocket
Firefox comes with build-in Pocket integration that can allow users to quickly save the article for a read it later function to easily find any articles saved in Pocket from various sources and devices.
Pro Easy screenshots without extensions
Within the browser itself, you can easily take screenshots and save them to your computer.
Pro Text-to-speech (with adjustable speed) without add-ons
Firefox Reader Mode includes Narrate, a feature that adds text-to-speech functionality to the browser.
Pro AdBlocker onboard
It's possible to add own filter lists. The internal adblocker is faster than any external.
Pro Compatible with Chrome extensions
Since Opera is built on WebKit, the same engine Chrome (among other browsers) is built on, it was possible for Opera developers to allow Chrome extensions to run in their browser.
Pro Video pop-out feature
You can pop-out any video and it will stay on top while you are surfing on other tabs.
Pro VPN onboard
Free and fast VPN onboard, includes Germany, Canada, USA, Netherlands, Singapore.
Pro Strong HTML5 feature support
Opera scores highly on HTML5 feature support.
Pro Battery saving mode
Pro Opera Turbo
Opera has a functionality called Turbo which increases browsers speed and reduces data consumption considerably by compressing the data that's found in a web page.
Pro Free and open source
Source code available on GitHub.
Pro You can install extensions on the sidebar
Unlike other browsers, you can install add-ons such as tabs, bookmarks, history, sessions, notes, and extension manager in the sidebar. So you have the flexibility to use it for your purpose.
Opera is full of user controlled settings. Everything can be imported and exported even RSS feeds. It can be skinned in a variety of different themes or it can be extended through the use of a large number of plugins available.
Pro Speed Dial
Opera's Speed Dial (new tab page) has the ability to hold as many websites as you want, and you can customize it almost however you want. You can also organize the websites on the Speed Dial into folders.
Pro Opera can have extensions from Opera Addons & Chrome Store websites
Opera supports cross-browser extensions. Opera Addons site keeps quality up compared to Chrome Store. Full of useful extensions, Opera also supports extra APIs like Speed Dial API, Extension Sidebar API and Stash API meaning wider variety of extension possibilities than in Chrome.
You can save the Opera's setup files on a USB hard drive and run its portable version anywhere.
Pro Opera Sync
Opera Sync allows you to synchronize your bookmarks, passwords, and more from any computer, phone, or tablet.
Pro For Opera 2017 & 2018 gonna be time of new
Opera got Reborn UI as the first implementation from Opera Neon concept browser. Snap -feature is next. Opera constantly improves its basic functionality - unlike Chrome.
Pro Fast video loading
Opera loads videos faster, making it useful for slow internet connections.
Con Unstable add-ons
E10s has already caused many extensions to not work properly. Also, in November, only add-ons with WebExtension API can be installed.
Con Doesn't care for its original guidelines/goals
Mozilla originally aimed to be the "good guys" with user choice and privacy in mind. Their current leadership cannot be trusted to hold those goals in high regard:
1) Added Pocket - a privacy data sensitive plugin, made it mandatory
2) Tried to sneak in advertisement as "drive-by hack", backpedaled unconvincingly once users complained: https://www.cnet.com/news/mozilla-backpedals-after-mr-robot-firefox-misstep/
3) Tried to randomly inject a small percentage of Firefox downloads in Germany with a data collecting plugin (Cliqz) that tech-savy Germans consider adware (no opt-out question asked) : https://bugzilla.mozilla.org/show_bug.cgi?id=1406647
Con Adds more and more bloatware its original target audience (a high percentage of Linux users) doesn't need
1) Pocket is bloatware on top of the sync option
2) Screenshot feature inside the browser - bloat (probably helpful for their helpline when users who don't know how to take screenshots are calling?)
3) Attaching Cliqz plugin for a few random users without notifying them -> unwanted bloat
Con It's bloated
Just remember that Firefox was created to create a simple and fast browser that would focus on web browsing. However, nowadays Mozilla is adding more and more extensions like pocket or screenshots to the browser(that could easily be done as optional add-ons) that will bloat it just like the old Mozilla Suite (nowadays Seamonkey).
Con Uses GTK on Linux/BSD
Thus makes the integration on non-GTK Desktop Environments very hard.
Con No support for ALSA since version 52
You must use PulseAudio if you need sound in Firefox.
Con Slow performance in OS X
Uses a lot of resources and feels slower than the Linux and Windows versions.
Con Heavy and consumes a lot of memory
Mozilla developers should think on this as the new Microsoft Edge browser steals less memory and it just pops up the moment you open it. In the case of Mozilla, you have to wait for after clicking it for opening it.
Con Sometimes stop working for few seconds
Sometimes stop working for 10/15 seconds, then starts working again. Inconvenient when looking streaming video on youtube or twitch
Con Styles HTML-forms with the OS/Desktop theme
At least on X11/Unix that leads to terrible issues, that makes some websites simply unusable for example: If you use a dark GTK theme that uses white text and come to a webpage that forces black text on html-forms buttons you will get black buttons with unreadable black text.
Con Poor enterprise support
Does not use standard management tools such as GPO or Profile management. No MSI.
Con Installs Addons with updates
Mozilla is installing/integrating addons with every update like the Mr.Robot promotion - it also has integrated Pocket that spams you evertime you open the browser or a new tab with partners of Pocket.
Con Switches from HTTP version of a page to its HTTPS version wtihout notification
even though the HTTPS page might have different content (yes bad website design, but if my browser does something automatically I don't expect, it should tell me).
Con Very very slow
Not only is it slow to load, but individual tabs take MUCH longer to load than any other browser out there.
Con Updates not in sync with updates of (key) extensions
A key extension can suddenly becomes useless because extension updates tend to fall behind the updates of Firefox.
Con WebKit/Blink is not customisable or lightweight
Opera is built on WebKit/Blink, which means it shares cons of Chromium: high RAM usage (Not lightweight), lack of customizability.
Con Uneditable default search providers
It can not delete or replace already built-in search services such as Yahoo, Google, DuckDuckGo, and Wikipedia.
Con Extension icons not directly moveable
While Opera is currently available gratis (without monetary charge), it is currently not libre (meaning that it does not allow users to view the source code used to create, to modify that code, or to redistribute modifications) and is therefor neither free nor open-source software.
Con Opera is owned by a Chinese Consortium
Who certainly need to conform with Chinese government requirements to let them infiltrate everything you do online…
Con Fewer extensions
Con Cannot select tabs
Unlike in Chrome and Vivaldi, one cannot select multiple tabs to move to another/new window.
Some buggy or confusing behavior with the bookmarks bar and moving tabs, and entire windows turning black after a while.
Con Basically a Chrome clone
The latest versions of Opera are basically Chrome clones, so they really don't add anything new.