You also need privacy.resistFingerprinting.testGranularityMask set to 4 for it to work.

https://old.reddit.com/r/firefox/comments/q9kql8/help_settin...

for some reason, neither of these work.

I've been trying to get slack to show the current time on messages.

does it just hardcode GMT?

have you tried closing the tab and restarting the browser? failing that, there might be some mechanism where slack saves your timezone in its user profile (or localstorage) and uses that even if the timezone reported by the browser changes