I know at least at the start some users were confused by the federation process and created 10+ accounts to “subscribe” to other instances plus there have been bots signing up through smaller instances without capcha or email verification so that might explain the discrepency
This update doesn’t seem to remove any add-ons specifically but turns them off on more sensitive websites if Mozilla has not reviewed them.
It does look like there is an about:config option to turn it off, however it looks like this is most likely to be for bank websites etc. and only for add-ons not reviewed by Mozilla.
Currently, at least, ublock origin and most of the bigger adblock add-ons are reviewed by Mozilla and shouldn’t be affected