I am stuck... first problem was that clicking on any URL in Outlook generates an error - General Failure in Outlook. Uninstalling LastPass fixes it. This lead me to try various unstalls - LastPasss, then FireFox... everything. Now, starting from a clean slate, if I try to install via the Tools/Addons menu... it seems to work... says install will complete after restrat of FF. Except it doesn't... no matter how many times I restart FF. Still says "Install will complete after restart" thing under addon manager.
So, let's try running the full installer instead. That works - but afterwards, the above Outlook error bites.
I tried going back to FF 3.6, since this all seemed to happen around the time I upgraded to FF 4.0 - no, same symptoms.
Tried turning off AV.
Tried running CCleaner, plus removed all traces of FF and Lastpass directories under Program Files.
Still the problem remains... actually, it might be two problems, but I suspect some registry weirdness might be causing both.