Mozilla Firefox crashed suddenly

Some minutes ago, as like all time, I am visiting websites with my favorite and king of the browsers- Mozilla Firefox. There are more then 20 tabs on that specific moment. Suddenly it crashed unexpectedly. I don't know what is the reason of this problem. Just after crash a dialogue box appear and it asking me to send a crash report to Mozilla foundation.

The dialogue box also said me that:

We're Sorry
Firefox had a problem and crashed. We'll try to restore your tabs and windows when it restarts.

and a check box behind a line, which says:

Tell Mozilla about this crash so they can fix it.

I mark the check box and click on 'Restart Firefox' button at the right bottom. All of my tabs restore successfully when Firefox restart. Thank you Firefox, I am happy with you since 2004.