In my opinion, what happened here is NOT normal.
First off, FF now checks its new versions for compatibility with existing add-ons, and normally advises the user of any conflicts it finds (before completing the installation)... and will show the "offending" add-on as being disabled.
In contrast, what happened today:
1) there was no warning/message from FF that an incompatible add-on had been encountered.
2) rather than merely showing the add-on as present but disabled, the best why I can describe what happened is that FF completely "swallowed" the add-on... it's gone from within FF! Interestingly, Avast was still showing it as being installed. Moreover, uninstalling and reinstalling it doesn't work... it will NOT show up in FF at all [i.e., not even as being there but disabled].
3) and its absence is not just a mirrage/oversight, as the add-on no longer works in FF.
The fact that AdBlockPlus may be more comprehensive, or work better, does not mitigate what happened here.