hi, how are you opening firefox - through a shortcut on your desktop?

Yes, I start it through either a shortcut on the desktop or on the start bar

can you right-click this icon, go to properties and in the "shortcut" tab, look at what's entered into the path field? does it end in firefox.exe or are there some commands like -js-console appended to it?

Hi

Path just includes "firefox.exe"

++

Worth mentioning that this "browser console" only started appearing after I registered to "firefox monitor"

hm ok, does it change when you launch firefox in safemode once to rule out the influence of an addon?

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

In safe mode the "browser console" does NOT open.

in this case please try to disable your few addons in the firefox menu ≡ > addons manager > extensions panel & then reenable them one by one, restarting the browser in between to hopefully narrow this down to a particular one.

a helpful helper has given me the hint that it's probably "RasTeck TTS Reader" causing this, since some reviews on the addons page describe a similar behaviour: https://addons.mozilla.org/firefox/addon/rasteck-tts-reader/reviews/

Thanks

Found the culprit: It is "Rastec TTS Reader" addon.

Do you know of any other reader which works with firefox?

the reader mode feature in firefox would have a narration feature built-in: Firefox Reader View for clutter-free web pages

otherwise there are a handful of other tts addons available on addons.mozilla.org, but i don't have personal experience with them. https://addons.mozilla.org/firefox/addon/ttsfox/ seems to be the one with the highest ratings.