Comment 46 for bug 861664

I did some testing and can confirm that uninstalling another extension resolves the problem - as does any other change to your extensions (enabling/disabling an extension, updating some extension etc). The add-on isn't really uninstalled and stays in the extensions/ directory, the update is actually applied correctly. It is only that the add-on manager temporarily "forgets" about it. I also verified that a minor browser update fixes the issue automatically - it forces the add-on manager to check the list of add-ons again.

So far I have only seen one report of this issue (https://addons.mozilla.org/addon/adblock-plus/reviews/313717/), no idea how many Adblock Plus users are affected. Is removing the update from AMO really the best course of action? I am pretty certain that I will receive lots of complains about Adblock Plus not being compatible with Firefox 7 then - install.rdf of Adblock Plus 1.3.9 lists Firefox 7.0a1 as maxVersion which means trouble even if AMO says something different.