hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Your suggestion worked... so I disabled the extensions one by one until the issue cleared; then I re-enabled all but the apparent offender -- narrowed it down. The culprit is the Kaspersky add-on. It was triggering the error when my mouse crossed over certain active elements, making the popup appear to be somewhat random. Hopefully, they'll get a fix out soon.

Thanks for the help, Philipp. I should have figured this out on my own, but it's been one of those days ;)

and sorry for misspelling your name, Philipp.

no worries, glad you could get it sorted...