If an installed theme was causing the problem, it may have an update available that will fix it.

Click on the Tools menu and then choose Add-ons. The Add-ons dialog appears.
Click the Themes panel, which displays the list of installed themes.
To check for updates for installed themes, click Find Updates.
If updates are found, install them by clicking Install Updates.
When the installation is complete, click Restart Firefox.
After Firefox restarts, your themes should have been updated.