Hi again, I'm back..
It's past March 15, and i just wanted to update my situation about the slack and firefox. it seems i can use it just fine -- and i did not download any plugins or stuff yet still.. but.
I found an article on a website that mentioned you can change your User Agent in something called "responsive design" which is in firefox natively
Tools => Web Developer => Responsive Design
and the box is at the top of the screen
So i google "my user agent" which says
Mozilla/5.0 (Windows NT 5.1; rv:47.0) Gecko/20100101 Firefox/47.0
Slack starts with that 'were sorry browser not supported, upgrade now" ...so i change the two 47.0's to 60.0 ...then click the X button in the top left corner of the screen, and slack loads normally and works fine now.
So i'm doing that, but occasionaly it still resets back showing that upgrade page...so then i have to open the response design tool, and paste in the 60.0 hack again...then it loads just fine..
I dont know how often or when it happens, i think the page refreshes itself once a day or something, i dont know.
it's no big deal really, so hopefully this does NOT actually get patched and this trick continues to work for the foreseeable future.
i did not actually do the "about config" mentioned in the 1st post of this thread, instead i did "responsive design" which applies only to just 1 selected site. cause i dont know if the about:config thing applies to all websites globally, i only needed for just slack only, nothing else.