Where can I find a Mozilla Firefox crash log?

You can access the submitted crashes reports by typing about:crashes on the address bar.

The MozillaZine Knowledge Base has an article about this subject.

The log report files are located at %userprofile%\AppData\Roaming\Mozilla\Firefox\Crash Reports\pending on Windows.

Please note that the DMP files appear to be binary in nature.

These crashes might be caused by some extensions; try to run it in Safe Mode and see if you get the same problems.

If Safe Mode works correctly, then most likely it is an extension fault; restart it in normal mode, disable all extensions, and restart again; if it runs smoothly, then the crashes were caused by one extension; try to re-enable one at a time, always restarting, and when you get again these issues, you have located the problematic one.

If it doesn't solve the problem, or if you want to learn more about the subject, read also the knowledge base articles on Firefox crashes and troubleshooting.