I've also tried creating a new profile and restarting with add-ons disabled. Nothing fixes the problem.

I can copy text from anything else and paste it into Firefox.

When I select text (or the URL) and copy it I can't paste it into Firefox or anything else. The clipboard is empty. If I right click on a field the "paste" option is disabled.

I've tried the control-c shortcut, right mouse button copy, and menu edit copy. Nothing works - the clipboard remains empty. If the clipboard contains text and I try to copy something new from Firefox, the clipboard is emptied.

Try to uninstall and reinstall Firefox as a normal user in case there are permission problem with the current installation.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Also check for UAC issues.

• https://support.mozilla.org/en-US/kb/windows-administrator-launcher-process-error-fix

Try installing Firefox from the Microsoft Store.

This has been an unresolved issue for a number of months now. When selecting text on webpages and right clicking, the copy option is randomly greyed out 50% of the time regardless of website or extensions. The only workaround is to click in the address box and then reselect the text. If the recommended action is asking the user to reinstall or even worse resort to use the ms store, then it would appear that Mozilla is suggesting that the user take ownership of the problem rather than investigate and fix the the issue. Might be a sign to move onto a browser that does not have this bug such as Chromium or Thorium!

Doesn't the keyboard work as well?

Try keyboard shortcuts in case the buttons on the webpage or other methods aren't working.

• Copy: Ctrl+C or Ctrl+Insert (Mac: Command+C)
• Paste: Ctrl+V or Shift+Insert (Mac: Command+V)
• Cut: Ctrl+X or Shift+Delete (Mac: Command+X)

Make sure you are not running Firefox in compatibility mode or as admin. Also make sure your system has UAC enabled.

It would be helpful to know if it still happens with Firefox installed from the Microsoft Store.