I am just setting up Fedora Core 3 on my Toshiba A30 laptop, and I am very impressed so far.
Gnome and KDE have Firefox as the default browser and I thought I would list the extensions I use in Firefox since I am setting up a clean install.

Tabbed Browsing Extras.
Spell Checker for Text and forms.
Web Developer Tools.

A very good thread with lots of other extensions.