Mozilla Firefox Crashed

It happened twice tonight while I am browsing the net, Mozilla Firefox crashed and the windows were gone. I don't know how many of you out there who experience the same. I have to re-start the computer to go online again. I want to search about this tomorrow. I am not a computer expert and I am just afraid if this will happen all the time. I hope not. I should make a screen shot of the full message. Anyway, here are some the words that appeared on my computer screen as the problem arised tonight.

"We're Sorry

Firefox had a problem and crashed. We'll try to restore your tabs and windows when it restarts.
To help us diagnose and fix the problem, you can send us a crash report."