When I install an add-on to Firefox, choosing it from the ones I find by using the Tools / Add-ons menu, sometimes after a few days Avast pops up a warning that the add-on is "unreputable" and strongly recommends and offers to remove it.
Of course malicious add-ons could potentially steal my passwords, intercept anything I type in that browser and do horrible things in general, so when I see such warnings I don't think twice and I just remove the add-on, but it's becoming too frequent and annoying, and it surprises me a lot that apparently Avast knows so much better than Mozilla about unreputable Firefox add-ons.
If Avast discovers malice in an add-on and lets the world know and even offers to remove it, why at the same time Mozilla happily keeps offering the add-on for long time? I had some that Avast recommended me to remove many months ago and are still offered in FF. Mozilla might even offer add-ons that were never checked at all, see this answer, but how is that possible if add-ons are potentially so dangerous? I'm also curious about how common successful attacks via FF add-ons are.
Also because if it's so common that FF add-ons that keep being offered in FF are unreputable, I would have a good deal of worrying to do for the past, because I kept "unreputable" add-ons installed for a while before Avast found them.