Of these, Undo Close Tab is the one I miss the most after every fresh new Firefox install. Always a shock if I find out too late I've forgotten to install it... :)
Add Bookmark Here is a recent discovery that I'm liking a lot. It really takes the frustration out of trying to add a bookmark to a specific (sub)folder.
brentashley on NS4: "Okay, I'm tired of being polite about it [...] I'm not listening LA LA LA LA LA."
Trying out K-Meleon once again... Much improved since last time - and very tweakable.