The template "crashsig" does not exist or has no approved revision. arena_run_reg_alloc
This crash is much more common in Firefox 3.0.x than in Firefox 3.5.x. If you're using Firefox 3.0.x, update to the latest version. For instructions, see Update Firefox to the latest version.
For troubleshooting other types of Firefox crashes, see Firefox crashes - Troubleshoot, prevent and get help fixing crashes.