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. It says "There is no time yet recorded today". This is your first visit on this site today. I have at least 10 tabs on this site today. It says that a possible cause is that I didn't use my computer for enough time today, but that's not true. Whatever the duration, it shows the same thing. And on the site: "There is no time logged for this period".

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

doesn't work

Simply does not work. Very disappointing for paying members.

Still doesn't work on Firefox Developer Edition. The *title/name* of the page is shown individually, but the *site address* isn't actually identified! All my activities are lumped together under "Firefox Developer Edition". The customer support has also been totally unhelpful on solving the issue.

Strangely it does work on Firefox.

Update Dec 2018: The bug still isn't fixed after 7 months, which is pretty bad.

Don´t use this addon, mines your data as you can read in it´s policy.

Excelente aplicacion que me ayuda a ser conciente en ue uso mi tiempo

After a long time, I can finally confirm it is working on macOS and Firefox with a cookie whitelist (cookies disabled by default, only user-specified pages are allowed to set cookies). Thanks to the developer to create an actual support document for this edge case.

If you are a Firefox user who likes to whitelist cookies instead of accepting everything by default, here is your guide:
http://rescuetime.helpscoutdocs.com/article/90-the-rescuetime-firefox-add-on-does-not-work-with-my-custom-privacy-cookie-settings-in-firefox