Update: as requested: Unread Tabs extension

Saw this tip over at the MozillaZine Knowledge Base:
Change the style of tab markers.

Adding those CSS rules to your userChrome.css file will italicize the titles of tabs that you haven't read.

Screenshot of unread tab being italicized in Firefox

Very useful for keeping track of what you've read and what you haven't (also helps prevent you from accidentally closing an unread tab).