IIRC, I have read several other posts about the "Lightning" extension and serious problems. Do you absolutely need it? Are there no others with similar functionality??
FF lock-up of the whole GUI is not unknown. I've seen it many times.
Rather than a hard re-boot, try:
ALT-CTL-F1 (or any F1 through F6) to get to a virtual terminal, log in as the user who started FF, then command:
The return to your GUI session with: ALT-CTL-F7
If it works, FF will be gone bye-bye, and everything else will be okey-dokey. Worked well for me when needed.