hi, this will be caused by one of your installed extensions - unfortunately the internal id of installed webextensions is randomized, so it's not possible by afar to tell to which one that's relating to.

Disable or remove Add-ons

Yes, this "moz-extension:..." looks like a hint.

BUT

• Why now? I use just a few and well known extensions. None of them is newly installed.
• Why is the message quite polite, but does not mention it's provenience?

perhaps the addon received an auto update with this new functionality - i agree that the dialog should make its origin clear. if you find out which addon might cause this, it may be something to report to the reviewers at addons.mozilla.org...

Upps, you're right.

Just recognized, that my main FF installation has inherited a new extension via Sync from an other device.

It was a trial (and fail) of Deutsches Rechtschreibwörterbuch von ValeurAddons

I got no more fishy popups after I deleted this add-on

BTW, Thank you Philipp

Under 'Bewertungen' it now says :

"Hatte nervige Popups ohne Angabe der Quelle. Nachdem ich dieses Addon rausgeschmissen habe, waren auch die Popups weg. Vgl. https://support.mozilla.org/en-US/questions/1204254 "

Good for you to make others aware of this  !