My Mozilla Firefox crashes frequently (and even major updates don't help). I'd like to investigate the problem. Where can I find the log/report files? There seem to be no option to view the actual crash report when the browser crashes and offers to send a report to Mozilla.
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.
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.
%APPDATA%\Mozilla\Firefox\Profiles\<profile-id>\crashes\events (I guess they moved the location somewhen since nc4pk answer)
You may also be interested to these variables, for further tinkering.