Thanks Asyn, but I don’t believe this is a virus issue - I’ve already scanned with Malwarebytes and it found nothing, it seems more likely to be an issue between Avast’s Bad Browser Add-ons function and an add-on packaged within the new version of Firefox.
It would also help if Avast gave more information on why it felt the add-on was poor - just what is the rating based on?
I will try the advice to - “Those who wish to disable Firefox Monitor altogether can do so by going to about:config?filter=extensions.fxmonitor.enabled and setting the preference to False.” - and see if this resolves the issue with it and Avast.
I’m not among the US residents that are included, as ‘extensions.fxmonitor.enabled’ does not show in About:Config. Some would consider this a privacy issue.
As an update, I changed the language in Firefox to EN-GB from EN-US (something that I should have done a long time ago), and that seems to have excluded me from the trial - no issues with Avast scans after doing that.
It may reoccur if Mozilla decide to do a global rollout, but hopefully Avast will be aware by then and the issue will be resolved.