Hi,

it’s important that you disable all extensions and check if that solves the problem. or you can check it by refreshing Firefox. By using this option Firefox will be restored to default and all your extensions will be removed, but your personal data, such as bookmarks and passwords will be saved. To refresh Firefox, do the following: Open Firefox and enter about:support in the address bar. Press Enter. Now click Refresh Firefox button.A dialog box will appear. Click Refresh Firefox again.Your browser will now automatically restart. After the refresh process is completed, check if the problem with your keyboard is resolved.

Thanks.

You can 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

You can boot the computer in Windows Safe mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10