Maybe wrap Firefox startup in a shell script that backs up your profile data before starting the browser, and then when the problem occurs send a bug report and work with the developers to give them the information they might need.

These bugs suck for everyone and I'm sure Mozilla developers would appreciate any help they can get to track it down.

Have you tried hitting Ctrl+Shift+N to restore the previously closed windows?

These will behave differently: (1) "Quitting" Firefox will multiple windows open. (2) Using your WM to close both Firefox windows. The latter will cause only the last-closed window to be restored, but you can pop the others off the stack too with Ctrl+Shift+N.

Just so you don't feel crazy, I've had this happen multiple times as well (on both Linux and Windows).

In fact, it's one of the larger reasons (tabs constantly crashing is the largest one atm) I leave Firefox for side jobs instead of being relied upon as my main browser.