My friend Jon Christopher compares using Safari 3.1 and Firefox 3 Beta 4 as his primary browser and as a browser for developers to use to build their sites. He once again confirms that Firebug, a plugin for Firefox that aids developers in debugging their code, is the main reason many developers use it as their primary browser. If it wasn’t for Firebug I wouldn’t even have Firefox installed on my Macintosh.
(Source: Comparing Safari 3.1 and Firefox 3 Beta 4 – Monday By Noon)