I've
blogged before about the
sessionsaver add-on for Firefox but I didn't mention one of it's best features called SnapBack Tab. Basically, it's a menu item that gets added to the tools menu with a popup that contains a list of the last 10 tabs that you've closed. So, if you've accidently closed a tab all you need to do is select Tools|SnapBack Tab|a) <tab name> and voila your tab is back with it's entire browse history and everything.
In case you missed it, here is a post that lists my favorite Firefox extentions.