Mozilla Firefox was introduced in 2004 and hit 100 million downloads in less than a year after launch. However, things began to go downhill for it after Google Chrome was launched and heavily promoted in lieu of Firefox.
Still the default web browser on virtually all Linux distros, Firefox, at one point, had over 350 million users. But its market share has now dropped significantly, pushing it to the fourth spot behind Chrome, Safari, and Microsoft Edge.
