Mozilla Firefox is the second most popular web browser, after Google Chrome (21% market share as of June 2015), which is quite impressive for an open-source project born from the battered corpse of Netscape after it was defeated by Internet Explorer, then stripped and eaten by AOL.
After almost two years since its initial 0.1 release, Mozilla Firefox, or just Firefox, launched version 1.0 on November 9, 2004. Versions 2, 3, and 4 came out in 2006, 2008, and 2011, after which they changed the criteria for being a new version and started to roll out new releases faster. As of May 6, 2019, 66.0.4 is the official released version for desktops.
Firefox started its life in September 2002 as an experimental branch of the original Mozilla Suite (since discontinued in favor of the community-maintained SeaMonkey) called Phoenix. The Phoenix name ran into trademark issues with Phoenix Technologies, so it was discontinued in favor of Firebird. That name, in turn, was criticized by the developers of the free/open-source Firebird database server, even after Mozilla insisted their browser be called Mozilla Firebird, thus necessitating a second name change. Regardless of what you call it, the browser is mostly known for being hugely and relatively easily customizable through the use of add-ons and scripts, something it inherited from the Mozilla Suite. The email component of the Mozilla suite was refashioned into Firefox's companion, Mozilla Thunderbird.
In recent years Firefox has attempted to expand beyond its' core browser into other devices powered by their technology, with Firefox OS, but ultimately it was discontinued.
In November 2017, a revamped version of the browser known as "Firefox Quantum" was released, intended to streamline the browser for the modern era. However, this did not go without controversy, as many well-loved add-ons were deprecated by the new code.
For more detailed information, see its page on The Other Wiki. The browser's official webpage can be found here. In addition to the desktop version, Mozilla has also released a mobile version for the Android operating system, which can be downloaded from the Google Play store, and another version for iOS, which can be downloaded from Apple's App Store.
You don't actually have to be able to think in Russian to use it, though you might think so if you confuse the browser with the 1977 novel or its 1982 Clint Eastwood film adaptation.