To fix this I needed to delete some stuff, and allow Firefox to recreate:
- Open your profile folder:
Click Help > Troubleshooting Information.
Look for the section "Profile Directory" and then click "Open Directory". This open the folder where your profile is stored.
- Quit from Firefox
- In the profile folder delete these files:
- Restart Firefox. You might be asked if you want to enable some extensions, so choose what you want to do and then restart if required