Does it still happen in a new profile (without signing into sync account)? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Thank you for your response. I was unable to perform any of the suggestions regarding profiles. But what did work was to shut everything down and restart in Safe Mode. Then when I started Firefox, I finally was able to see the error screen it had been trying to display when it was crashing. That screen said: "Sorry. We’re having trouble getting your pages back. We are having trouble restoring your last browsing session. Select Restore Session to try again." And "restore session" brought me back to a normal website window. Then I restarted the Mac into normal mode and Firefox is now functioning normally.