Recs.
Updated
Mozilla Firefox is a free and open-source web browser developed for Windows, OS X, and Linux, with a mobile version for Android.
SpecsUpdate
Pros
Pro Strong HTML5 feature support
Firefox scores strongly on HTML5 feature support.
Though not as strongly as Chromium/Chrome browsers do.
Pro Strong developer tool
The built-in developer tools have been merged with the popular FireBug extension since FF57.
Pro Uses less resources
Firefox 57 (Quantum) and newer uses less resources than ever. It is proven with benchmark done by AppleInsider.
Pro Dark theme
Beyond the toolbar and tabs, it darkens UI elements such as the URL-bar, pop-downs, new-tab page and more.
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 Integration with Pocket
Firefox comes with built-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.
Cons
Con Cannot directly translate page
Unlike Google Chrome, if you visit a website with a different language, you cannot translate it, which is a bad user experience for some.
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 every time you open the browser or a new tab with partners of Pocket.
Con GTK Themes styles the HTML forms
If you're in Linux and 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 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
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).
Con It's a memory hog even though Mozilla claims it is not
Mozilla claims it's using 30% less RAM than Chrome but in real life tests it uses much more.
Recommendations
Comments
Flagged Pros + Cons
Pro Android version allows installation of addons
Unlike other browsers, Mozilla uses almost the same codebase as with the desktop version so extensions work as-is without code modifications - something other browsers cannot do due to their breaking and sometimes unrelated code branches to the mainline desktop branch.
Pro Easy screenshots without extensions
Within the browser itself, you can easily take screenshots and save them to your computer.
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 Mozilla pushes political messages through it
Everyday on the standard page, there is a new message. Sometimes this message can be political (when this happens, it is usually a left wing political sentence).
Also Mozilla made its cofounder and creator of javascript reseign because he was against homosexual marriage. He now develops a browser based on Chromium.
Con Experiments
They run experiments with users as a form of data collection.
Con Mozilla once partnered with George Soros to fight misinformation
Mozilla once partnered with George Soros to fight misinformation.
Con Promotes a political agenda
Promotes a political agenda in your browsing experience via Messages and Pocket recommendations
Con Supporting Gecko makes no sense
Since Blink is available under a more open and permissive license.
Con Google obsession
Lately, Firefox has been partnering with Google, leading to some privacy concerns.
Con Multi-user requires commandline
to use multipe profile in firefox, you need to run -no-remote switch command on firefox properties
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 No status bar
There's no status bar. Granted, there is a status tool-tip. But what if you want an actual toolbar, you're out of luck.
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 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 your browser does something automatically you don't expect, it should tell you).
Con Uses extremely old browser version
Firefox uses a 15-year old version of Netscape, which has been dead for years.
Con Impossible to launch two separate instances at the same time
E.g. one installed surf-Mozilla and another portable Mozilla for secure browsing (e.g. solely used for internet banking). One first has to close one to open the other.
Con Now a Google Chrome clone
Has lost its own way and is now following Google. It has copied Chromes behavior , its stupid-proof design and now its extension system.
Out of Date Pros + Cons
Con Can't scroll tabs with mouse wheel
It's pretty basic but you can't switch between tabs back and forth using your mouse wheel. Any add-ons supporting this have been deprecated since FF57.
Con Cannot open multiple instances
You cannot open multiple Firefox windows at the same time.
Con Slow performance in OS X
Uses a lot of resources and feels slower than the Linux and Windows versions.
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 Poor performance on SSD even without extensions
On a fast SSD such as Corsair Force LE series this browser still performs as if it were installed on a regular hard drive disk, meaning it takes long time to start, even without any extensions. On that matter Google Chrome is light years ahead of Firefox. This applies only for Firefox versions below 60!
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 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 No support for ALSA since version 52
You must use PulseAudio if you need sound in Firefox.
Con Installing some extensions still requires restarting the browser
While some of the developers already allow users to install their extensions without the need to restart the browser, not all of the extensions have been updated to support non-restart installation yet, as some developers have yet to add special code to do so.
Con Poor enterprise support
Does not use standard management tools such as GPO or Profile management. No MSI.
Con Gets slower with every extension
Con update: the following applies to Firefox versions 59 and below. Since version 60 this problem was fixed.
Until the 10th extension it works fine but after that it suddenly becomes slower and slower and at some point it takes forever to start - if it starts at all.
Con Icons display blurry on HiDPI
Firefox has not yet addressed its scaling issues for HiDPI screens, making the icons blurry.
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.