This would appear like a bug in your custom theme's gtkrc. Most likely Firefox simply didn't trip it before but now that GTK integration has been further improved Firefox attempts to use that portion of your theme.
** Changed in: gutsy-backports Status: New => Incomplete -- firefox 3.0-b5-ppa segmentation fault https://bugs.launchpad.net/bugs/212694 You received this bug notification because you are a member of Ubuntu Backports Testing Team, which is subscribed to Gutsy Backports. -- ubuntu-backports mailing list ubuntu-backports@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-backports