A couple of months ago, I started switching to Mozilla from IE. There are a few reasons for that, the biggest being how wonderful middle-clicking to open new tabs are (I tend to browse quite a few at the same time...), as well as how poorly IE reacts sometimes, also slowing the rest of the Windows Interface (apparently the worse side of browser/OS integration ).
The numerous (and serious) security issues were also a factor. AFAIK, there is still an open Javascript program that allows anyone to install spyware.
Last week I tried Firefox, and it's even better than Mozilla. The interface is superior, and it feels a bit faster, too. The extensions available are great! I installed a lot of them. For example:
1) Adblock : Blocks ads using Regular Expressions. Some ad heavy pages, such as Yahoo, look so clean they seem weird
2) Googlebar (although the cute Google/etc search on the right of the toolbar cover most of my use). It does have a keyboard shortcut for showing up or not...