It is possible that what you see is coming from a Windows feature (setting), i.e. this part is transparent in Firefox.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

thanks a lot I'm gonna try that

I didn't find out where from comes the issue. What do you mean when you talk about "userChrome.css" I never created that file. Does it pre exists and can be corrupted? ... ... ... Okay, so : I eventually managed to get rid of the yellow border. Hooray! I decided to completely uninstall Firefox, but a simple "Refresh" has been suggested, I went for that refresh. It solved the problem. I was fed up o that stuff :) This posts will may be help someone else one day :)

The yellow thin border came back. I give up for tonight :(

It is possible that what you see is coming from a Windows feature (setting), i.e. this part is transparent in Firefox.

Check your Windows settings. This is Windows 11 -

Thank you, I eventually got rid of that thin golden yellow border, after : 1 Uninstalling Firefox. 2 removing the Firefox folder in APPDATA All is explained there : https://support.mozilla.org/en-US/kb/uninstall-firefox-from-your-computer

Since that it's Okay. I launched a deep antivirus scan. (My security suite as a name beginning by a K.) It found and put on quarantine one Troyen that was located in my Download folder. I installed "Malwarebytes free" and launched a research that deleted some suspect files. It was late at night yesterday. It's now 8:40 PM here, in Paris France area. Thanks again :)