I have this problem as well and it's annoying. It's most easily reproduced on Tumblr for me. I also managed to capture a video clip of one of the incidents, but not sure if I can share here.

My Firefox is updated to the most current version (110.0.1 (64-bit) on Windows 11), the error was reproduced in private mode with my extensions turned off, and uninstalling, reinstalling, + refreshing my firefox application did not make a difference.