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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 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 Uses less resources
Firefox 57 (Quantum) and newer uses less resources than ever. It is proven with benchmark done by AppleInsider.
Cons
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 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 Many Addons are not updated/adapted for the new Quantum version (version >=57)
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 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 Google as its default search engine
Which is pretty ironic for a browser that's focused on "privacy".
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 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.
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.
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.
Pro Uses less resources
Firefox 57 (Quantum) and newer uses less resources than ever. It is proven with benchmark done by AppleInsider.