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

Simple, concise, works well, helps me saving time

Works well in standard Firefox Quantum 58.0b3, but not track sites in Firefox Developer Edition 58.0b5. I tested addon on Kubuntu 17.10.

Since Firefox 57 I've switched over from Chrome. This extension initially did not log correctly even though the notes here said it added support for Firefox 57. However, I recently received an update to my rescuetime OSX app and now everything works perfectly as expected. Thanks rescuetime. This is now a 5 star extension.

Best way to keep track of what you actually get up to on the internet all dang day - warning: may be eye opening.

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.

Good overall addon, and now works with Firefox 57+

can't track browser time

Causes problems with e10s and will stop working in Fx 57, need to be ported to WebExtensions.

After install, icon is not showed on the bar. One need to go do adjusting options and drag icon onto the bar. Then click it and it works.

If you're encountered the problem mentioned in a number of these reviews where you attempt to login to the RescueTime add-on and you can't type in the email field, the work around from the add-on developer is mentioned here ( https://addons.mozilla.org/en-us/firefox/addon/rescuetime-for-firefox/reviews/804290/ ) stating "We've been able to reproduce the issue. As far as we can tell, the problem with not being able to interact with the window ONLY happens when the RescueTime icon located on the menu panel (under the hamburger icon on the far end of the toolbar). If the RescueTime icon is directly on the browser toolbar, the window works. While we work on a fix, the workaround is to move the icon to the browser toolbar. If you do not want the icon on the toolbar, you can put it there temporarily, sign in, then move it back to the menu panel and the site tracking should work just fine."

Just like the others complain... it does not pop up after install.

Glitch in the system and I am surprised that the DEV with such low ratings.. is not fixing this issue.

It causes issues with other extensions (Tab Groups in my case) because it has not been updated to work with multi-process Firefox.

After adding the extension button to the toolbar I was able to click the button, log in, and start tracking.

My only wish is a, user definable, option to combine websites and apps as one. For example, I'd like messenger.com from the web and Facebook Messenger from Android to be counted as the same RescueTime Activity.

It works just fine. Remember to put the icon somewhere visible after installing by going to "customize" on Firefox. Signed in without issues.

Would be great if it worked. It didn't.

signup doesnt work

This addon is extremely useful to allow me to track my internet usage with RescueTime. I do not have any customisation issues with the addon and it seems to stay logged in. Hopefully more users will update/post their reviews. Make sure you get the latest Firefox version and keep your addons up-to-date to avoid issues.

The add on is quite good. I didn't encounter the customization issues and the add-on didn't change anything.

The setup was simple and it has become an essential productivity tool for me.