In early 2006 I first installed Firefox on my laptop. The reason being that Internet Explorer could not emulate mobile sites the way Firefox could. (I worked in the mobile business at the time and really needed that functionality.
The mobile browser emulator was an Add-on (extension) to Firefox which had to be downloaded and installed from the extensive Add-on directory.
Not only was Firefox faster than IE, but the above mentioned extensions completely blew me away, my personal favorite being the Foxmarks Bookmarks Syncronizer which ensures that my different machines all have the same bookmarks.
Firefox website: http://tinyurl.com/y3prgo
Some external reviews: