This shouldn’t happen normally, but I could suggest a few basic things to try:
- Upgrade your Firefox in case it’s outdated
- Don’t log out on the website (stay logged in because the add-on uses the same login)
- Disable all other extensions to try if another extension is causing something bad
- If you are using some sort of crazy security settings in Firefox, try resetting them to the default
- Clear your browser cache by pressing Ctrl + Shift + Del
It works for most people, so there might be something special about your setup.