Running Firefox 3.6.12 with Adblock Plus this morning - Adblock prompted me to update, so I did - after Firefox restarted all functionality of Adblock Plus was lost! 
Completely uninstalled Adblock, reinstalled - nothing! Read on Adblock's forums, and plenty of other people are having the same problems.
Completely uninstalled Adblock, reinstalled - nothing! Read on Adblock's forums, and plenty of other people are having the same problems.