I'm a Windows XP guy, but I know there are a lot of Mac users who prefer Firefox. I certainly prefer it over IE7 and Opera. You can configure Firefox to do things that no other browser can do. The options you have with Ff are almost endless.

Just for shits and giggles, what extensions does everyone use? Mine are:

1-ClickWeather
Adblock Filterset.G Updater
Adblock Plus
All-in-One Sidebar
Customize Google
CuteMenus - Crystal SVG
Download Statusbar
Gmail Manager
Go to Google
Google Toolbar for Firefox
gTranslate
Image Zoom
InForm Enter
MR Tech Toolkit
NoScript
PDF Download
Screen grab!
Searchbar Autosizer
Tab Mix Plus
Update Notifier