I understand the frustration you're experiencing with the saved windows not opening in Firefox after the update. Let's try a couple of things to get them working again. First, make sure Firefox is fully closed, including all background processes. Next, try restarting your Mac to ensure a fresh start. If that doesn't work, try opening Firefox in Safe Mode to see if any extensions or add-ons are causing the issue. To do this, hold down the "Option" key and click on the Firefox icon to launch it in Safe Mode. If the windows open in Safe Mode, it might be an add-on causing the problem. You can then disable add-ons one by one to identify the culprit. Hopefully, one of these steps will help resolve the issue and get your saved windows opening again. If not, please feel free to reach out for further assistance.

Thanks suram080 - I had tried restarting my Mac a number of times, but forgot about safe mode. I know you suggested restarting Firefox in safe mode but I tried restarting the whole computer in that manner, and that cleared up the problem. So far it's still working - thank you for the assistance!