Rated 2 out of 5 stars
Good news first: it is finally working! Just make sure you have the latest version of the Firefox add-on and the OSX client.
However, there are bad news for users like me: the add-on only works if you accept cookies in general (like it is the default setting in Firefox). That may not be a big deal for the vast majority of users, but it is a deal breaker for me. I only accept cookies from a specific list of websites I frequently use and require cookies. Even when you whitelist `*.rescuetime.com` to be able to set cookies, the extension doesn't work. The extension seems to require some other cookies or what not, but I'm not able to figure it out. I even debugged through the extension itself, but I gave up. Hope I'll figure it out one day...
To whitelist cookies for an add-on, you need to first get the add-on Internal UUID from 'about:debugging'. Locate the add-on in the list and copy the UUID to clipboard.
Try adding these to your whitelist:
moz-extension://xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx (the add-on UUID)
Should solve the problem :)