You appear to be using userChrome.css and userContent.css. You can set toolkit.legacyUserProfileCustomizations.stylesheets => false in about:config to disable these files.

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 if userChrome.css/userContent.css is causing the problem.

• switch to the Default System 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

You can set toolkit.legacyUserProfileCustomizations.stylesheets => false in about:config to disable these files.

None of that got rid of the overlay. I also tried temporarily running new, empty profile and it did not have the overlay but this didn't clarify much.

I'm running Leap and Tumbleweed and not able to replicate your issue on Distro, Mozilla, nor Waterfox. X11 or Wayland? Graphic driver info? lshw -numeric -C display You may have to install lshw or use infocenter to get the info.

Operating System: openSUSE Tumbleweed 20240226 KDE Plasma Version: 5.27.10 KDE Frameworks Version: 5.115.0 Qt Version: 5.15.12 Kernel Version: 6.7.6-1-default (64-bit) Graphics Platform: X11 Processors: 8 × Intel® Xeon® CPU E3-1535M v6 @ 3.10GHz Memory: 62.2 GiB of RAM Graphics Processor: Quadro P4000/PCIe/SSE2 Manufacturer: HP Product Name: HP ZBook 17 G4

None of that got rid of the overlay. I also tried temporarily running new, empty profile and it did not have the overlay but this didn't clarify much.

New profile is not showing the issue, sounds like an add-on issue.

I'm running Leap and Tumbleweed and not able to replicate your issue on Distro, Mozilla, nor Waterfox. X11 or Wayland? Graphic driver info? lshw -numeric -C display You may have to install lshw.

Well, good for you then. And I'm here have to write even this text behind a grey bar because it generated another one over the input window too.

Full lshw -numeric -C display shows whole bunch irrelevant BIOS-level stuff and your example is not from lshw's output at all. But my setup is typical X11/KDE with AMD Radeon RX580 and latest Mesa.

New profile is not showing the issue, sounds like an add-on issue.

Except that "Troubleshooting Mode" supposed to disable them all and it still didn't help. And what kind of addon would even be that? I'm 80% certain that some Firefox's (or some toolkit's used by it, like GTK) built-in rendering debug option got flipped somewhere. Maybe even one that is not triggered by prefs but by some hardcoded condition.

Okay.

Change apz.minimap.enabled to false in about:config. This setting had been broken for many years until it was fixed in version 123 (Bug 1604280).

Change apz.minimap.enabled to false in about:config. This setting had been broken for many years until it was fixed in version 123 (Bug 1604280).

Thanks! I have no idea how & when this got flipped and had no forethought to search for 'map' keyword too. It's been ages since I was so helpless with a technical issue.