Does it still happen with default settings in a new browser profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

Copy the sessionstore.jsonlz4 file in your profile folder to transfer across your tabs.

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 the "Refresh Firefox" button on 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

Thanks for the suggestions, but I already gave up and reinstalled (losing all of my ToDo list tabs).

I think there may be a limit. I have 60 windows and some significant number of tabs per window, and the open tabs list never appears on the Firefox View screen for me (that section is blank).

But if yours was working with all those tabs up to now, perhaps it is something else. Occasionally an open tabs seems to lose its identity, the URL is blank and reload doesn't do anything. Maybe you and I have one or more of those, especially among the older tabs?

Thanks for the suggestions, but I already gave up and reinstalled (losing all of my ToDo list tabs).

Normally reinstalling Firefox doesn't delete your open tabs (you can still use History > Restore Previous Session and History > Recently Closed Windows).

Or did you select an option to delete your data? If you decide you want to try to recover older data, we can give you some tips, depending on how you lost it.