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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.
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Pros
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 Strong HTML5 feature support
Firefox scores strongly on HTML5 feature support
Though not as strongly as Chromium/Chrome browsers do.
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.