If you wish to diagnose all profile-related issues (which include extension and theme issues), you can do so by creating a new Firefox profile (do not delete the original profile, just create a new one). Your added extensions and themes, bookmarks and other personal data and settings will not be present when Firefox starts with the new profile but will still be available in the original profile. If the problem still exists in the new profile, complete the remaining standard diagnostic steps.
If the problem is resolved in the new profile, you have two options:
1. You can migrate your data and settings by selectively copying your important profile data to the new profile, then reinstall your added extensions and themes or,
2. You can try to fix the problem in the original profile, as outlined below.