Very good! It even recognizes specific sites as different types. Like "Reference & Learning:
google.com, realpython.com" and "Design & Composition:
MS Paint" (MS paint from desktop). Really surprised to see someone has actually made this. One of those things i had actually hoped for and assumed would never exist.

Its a tragic they have taken so many low review.

This is really good addon but it slows down Firefox significantly. I had to test all my addons one by one to figure it out. Its a shame as the Chrome counterpart is fast. My usecase is heavy browsing (loads of tabs). It might be ok for light usage.

Works fine, but could have more integrated features such as category change when browsing pages, directly from the extension itself.

As others have mentioned, it needs cookies permissions:
https://help.rescuetime.com/article/90-the-rescuetime-firefox-add-on-does-not-work-with-my-custom-privacy-cookie-settings-in-firefox

Doesn't work with firefox developer edition. RescueTime shows everything under "firefox developer edition" activity without displaying visited websites.

Tested with firefox developer edition 64.0b6 and latest RescueTime 2.10.1.1355 under Ubuntu Linux 16.04 LTS

Worked like a charm!

It helps you be productive