The web 20 years ago was a dark and miserable place. Netscape was the dominant player and their Navigator product was clunky, with a very awkward rendering engine and a lot of proprietary web extensions.
Microsoft, never being one to miss a trick, launched IE4 in 1997 which in many ways was a superior product. It supported dynamic content a lot better than Netscape (still in a largely proprietary way), was faster etc. It was so integrated in to Windows that it could replace your entire shell on Windows 95 or NT4. Windows 98 continued this.
Anyway, whilst IE4 and later 5 were unstable, they were subjectively better and easier to obtain for Windows users. Netscape was such a mess that they gave up entirely on their code base and created the Mozilla project for a next-generation browser. Microsoft launched IE6 in 2001 with just the right mix of Netscape compatibility and proprietary (shiny) extensions that everyone went for it. At one point, IE had almost 90% market share!
With this dominant position, Microsoft basically gave up developing their clunky, insecure web browser as businesses flocked to make applications require it. The Mozilla project spun out of the AOL-owned Netscape and launched a niche browser 'suite' which included email and web page editing all built in. It was slow, buggy and bloated - but very standards based (contrasting to IE).
A group of people took the good bits from the Mozilla project (browser) and tidied up the extension engine. They called it Phoenix and added useful features like tabs, download management etc. This got renamed to Firebird and then to Firefox for trade mark reasons... The world was given a browser that could take on IE. On launch day they had elaborate marketing schemes like full page adverts in the press and heavy promotion via Google.
Mozilla alone created a product that could take on Internet Explorers dominance, forced Microsoft to continue to develop IE towards a more standards-focussed goal and empowered us users to get back the web.
As Chrome (and Blink/WebKit) become more dominant it's critical that we have choice. The web was a dark place with too many sites requiring proprietary Microsoft extensions just to run apps. Lets hope it never happens again!