Another user (mdontu) just detailed the same problem I have.

Usually this extension is absolutely great. I've been using it for years. However, for quite some time it seems to struggle with resetting large whitelists when Firefox is sync'd over multiple computers. Gets really frustrating when you have to re-whitelist sites that you know you have already whitelisted.

The problem seems to be getting worse. I think the developer may be aware of it, but this really should get addressed.