Just a quick note, I have very little time at the moment, but more later.
On the Firefox "Add-ons Manager" screen, I clicked "View Recent Updates" (from Options icon at top), and discovered that the 6 active addons listed as recently updated are exactly the 6 plugins I lost and reinstalled. I suspect that the addons listed by others are also very recently updated. Unfortunately, that may not be the whole story, as 2 disabled addons were lost in the earlier 'purge' and aren't listed, but that may be because I haven't reinstalled them, Sage and FreeHDsport (sic?). I don't use them currently. Does anyone know if they were recently updated?
Avast Cleanup has often complained about FreeHDsport, but I've never allowed it to proceed.