Interesting add-ons that I don't use or have not installed
More In Content UI in Firefox 4!
Displays a timeline of the current active tab's history in a sidebar.
MurdochAlert warns you whenever you visit one of the 100+ Murdoch Family-controlled websites. If you're not ready to block them all, MurdochAlert can warn you instead. Also it's handy for identifying news sources controlled by the Murdoch Family.
An extension replacing the right-click menu of Firefox with a doughnut-shaped menu.
Dormancy 'retires' tabs that have gone unused for a while, freeing up that memory. It then restores the tabs to life when accessed.
EXPERIMENTAL. MAY EAT YOUR SESSION OR DO OTHER BAD THINGS.