My newest favorite Firefox extensions are Tab Mix Plus & Menu Editor.
Tab Mix Plus
I've tried Tab Mix Plus (TMP) in the past and didn't really care for it. It had some default settings that I didn't like, the configuration was confusing (using terms I didn't understand out of context) and didn't do the one thing that I wanted, which was make double-click and middle-click on the tab/tab-bar open and close tabs.
But a recent release fixes all these issues. I still think it is a bit overkill and it could use little config GUI tweaking but it is definitely an improvement. It adds my double/middle click requirement (basically copying the Tab Clicking Options extension), lets me fix some of the tab changes in FF2 and adds some more great new features.
In addition to pulling in the Tab Clicking Options extension TMP seems to have added the Session Manager extension. I think this is great. Now instead of three extensions I only need one. Of course, FF2 added some Session management features but TMP's features are more complete.
The final great thing about TMP is that checks for compatibility issues with other extensions. After installing TMP (and restarting FF) I was prompted with a list of extensions that were incompatible (Tab Clicking Options was on my list) and then given a set of options on how to proceed. They've gone above and beyond where most extensions would have stopped. They don't just say "EEK incompatible" and quit. They say "EEK incompatible" and give me some buttons that will take care of the problem.
Great job and many kudos to Gary Reyes, oneman, and the other TMP contributors.
The other big annoyance that I've always had with extension systems (in any application) is that the extensions aren't held to any standard on menu items. I seem to end up with menu items all over the place, out of place, out of order, in context menus and on the menu bar. And to go one further 90% of the time uninstalling the extension leaves, the now old and broken, menu items.
So, in steps Menu Editor. It's not exactly intuitive but it gets the job done! My FireFox tools menu is clean at last. The 'Options...' item is finally returned to the bottom of the Tools menu. I have regrouped all my 'Extension X Options...' menu items together and removed all the other stuff I never use.
It even lets me fiddle with the context menu, getting ride of all the fluff there as well.
Menu Editor's behavior is not always predictable. Extensions still have power over their menus and can make menu items show and hide even if you change their state with Menu Editor. But it's better than nothing and some fiddling around will eventually get your menus where you want them to be.