Any messages in the Web Console or Browser Console (Multiprocess) that look relevant ? *https://firefox-source-docs.mozilla.org/devtools-user/web_console/ *https://firefox-source-docs.mozilla.org/devtools-user/browser_console/ You can create a new profile as a quick test to see if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles If the new profile works, you can transfer files from a previously used profile to the new profile, but be careful not to copy corrupted files to avoid carrying over problems. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

The font in your screenshot looks unusual. Does it still happen in a [[Profile Manager - Create, remove or switch Firefox profiles|new profile]] or after [[Refresh Firefox - reset add-ons and settings|refreshing your existing profile]]? Try disabling any third-party antivirus or security software and rebooting in Windows Safe Mode.