Hello Nguyen Vo,

Would you try this please :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar and look for the preference :

gfx.webrender.enabled and set its value to true

Then close and restart Firefox.

Is that box gone now ?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration 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 Safe Mode start window

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

If my first suggestion didn't make the box disappear; would you
please try this on the about:config page :

Type in the search bar acceleration

Look for the preference layers.acceleration.disabled

and set its value to true

(don't forget to then close and restart Firefox)

McCoy said

Hello Nguyen Vo, Would you try this please : Type in the address bar about:config and press Enter (promise to be careful, if asked) Type in the search bar and look for the preference : gfx.webrender.enabled and set its value to true Then close and restart Firefox. Is that box gone now ?

Thanks for great support. I've tried for three solutions :

+ gfx.webrender.enabled    =  true
+ layers.acceleration.disabled =  true
+ start in safe mode

==> they can all solve my problem. Thank you.

Could you please share the root cause? Will it have a fix on next update?

Nguyen Vo said

Could you please share the root cause?

In all honesty : I haven't been able to find an answer to your question, but maybe you could see if you need to update your graphics driver (?) At least that worked for some people here :

https://www.reddit.com/r/firefox/comments/cm3oxt/ui_glitch_black_box_in_the_topleft_corner/