If the problem persists with the default theme in use, you need to troubleshoot your extensions. If an extension is causing the problem may have an update available that will fix it.

Click the Tools menu and then choose Add-ons. The Add-ons dialog appears.
Click the Extensions panel, which displays the list of installed extensions.
To check for updates to each installed extension, 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 extensions should have been updated.