Normally when Firefox and other programs crash there's a prompt short after asking if you want to keep waiting. Did this ever show up? I'd be curious to see if the program shows up on the System Monitor or by running htop on the terminal...
You also mentioned that rebooting fixed the issue but have you tried to simply log out and log back in? In my previous comment I mentioned restarting the display manager but actually this is more like what I was referring to: Refresh Zorin OS desktop without logging out
Maybe you can try this and if it doesn't work, log out instead. But even if this does work, I must admit that I have no idea what could be causing that behavior...