https://support.mozilla.com/en-US/kb/Firefox%20crashes#w_get-help-fixing-this-crash

Type about:crashes in the URL bar and hit Enter.

1. Middle-Click the hyperlinks to bring up each Crash Report page in a new Tab; each page may take a few minutes to load.
2. Then copy'n'paste the URL of each of those reports into the message box here.
3. We'll see if we can help you figure out what is causing those crashes to happen.

Additional comment because I'm getting the same problem. I have the latest Firefox 11.0 Check on Help>About Firefox shows that I have the latest. Intermittent. Unable to predict when it will happen. Same website, sometimes will crash, sometimes not. Is having a critical affect when it crashes on secure sites.