On the Customize panel, or on other toolbars, the button may have a different appearance: instead of a plain + sign, a + sign on a tab. If all else fails, try the Restore Defaults button at the bottom of the Customize panel to restore the starting toolbar layout.

Ref. Customize Firefox controls, buttons and toolbars

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 "Refresh Firefox" in 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