Why
That's too classic, I'm not one of those to give up a program because of it's theme, but yet after a loot of customization I could not find it user-friendly. Add-on compatibility, The way menus and tabs works, problem with some plugins and some other problems.
Another reason is that I don't use mail client in windows and only use web-based email for all my accounts(they are linked together), so I did not need mail feature
, and for chat, I use that in web based client (meebo) so I did not IRC
... and last reason is that I don't like Mozilla anymore