Rated 2 out of 5 stars
Using the latest 57.0b9 on both standard and developer editions of Firefox for Mac, after installing the RescueTime add-on I can link it to my account just fine, and if I click on the icon in the toolbar the summary does correctly identify which site I'm on.
*But* it never logs any time on these sites. If I visit a site I've not seen in Chrome that day it always reports it as being my first visit. If I visit a site I've seen in Chrome it reports the previous total time spent there but never increments.
If you're using the add-on in conjunction with the RescueTime desktop app, you'll need to make sure you have the latest version of the desktop app for the recent add-on updates to function properly.