Edit: Also do you happen to have more than about 700 rows in the Options table? I found that there is a 100k limit on the synced storage which by my calculation will store about 700 rows. If that is the case then I will abandon the synced storage. The not synced storage is unlimited.
Edit: I will upload a new version. If that doesn't fix the problem then my conclusion is that the problem is with the Firefox synced storage. Are you logged in with a Firefox account? If so, do you use this account on multiple devices?
The new version 5.0 fixes some bugs. Please let me know if you still have problems with the new version.