Steps to replicate? Provide a link to a site(s). What security software are you running? Did you try Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search104

Are you running 10.13.6? I have a macbook pro from 2012 running the same version. Provide links and I can test.

I refreshed after starting in troubleshoot mode and finding the problem gone. I'll let you know if problem comes back. For now, it seems gone. I'll run a deep scan for malware tonight, but it seems like the Refresh may have done the trick. Thank you so much!

Great. Mark it as resolved and we'll leave the light on for ya.

What the Refresh feature does https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Perhaps an Extension you have installed in Firefox is doing this whether due to a bug or a recent update in extension.

Note the older Firefox 115 ESR is coming to an end soon as the planned update Fx 115.15.0esr in early September will be the last unless Mozlla decides to extend it with some more updates.

Pretty much every other web browser sopped support of the EOL Windows 7, 8, 8.1 in Jan/Feb last year and I believe the eol macOS 10.13 was dropped by Chrome/Chromium back in summer last year. Fx 116.0 and later requires Win 10/11 and macOS 10.15 or later to run.

The poster at https://forums.mozillazine.org/viewtopic.php?t=3124430 describe a similar thing of having a a small credit card size transparent window on upper right side of page. The OP figures it was due to either Ghostery Tracker or Ad Blocker extension.