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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 Free, libre/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 Organization that cares about your privacy
Mozilla is a non-profit organization that cares about your personal privacy. All other major vendors release a browser to boost some other market.
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 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.
Pro Great developer tools
The built-in developer tools have been merged with the popular FireBug extension since FF57.
Cons
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 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)
Recommendations
Comments
Flagged Pros + Cons
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 Strong developer tool
The built-in developer tools have been merged with the popular FireBug extension since FF57.
Con When you search in a website (Ctrl + F) there are no marks appearing in the right scrollbar
All Chromium based browsers have this feature.