Even though I sign in through the RescueTime's Firefox add-on, it constantly shows a red esclamation mark and sign-in prompt. It used to work great, if it's not fixed soon, I'll start looking for alternatives.

After recent update just asks me to login and red exclamation remains. Can't run desktop app because it has a runaway process that I have reported but they won't fix. What to do?

Not working in Android

Can get your data. What's the need in that?

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