While i'm also very concerned about color switchboard plugin being gone (i'm a photographer so i make a substantial use of color profiles) i did some digging and found this:
Seems like Elementary wants to move setting color profiles from "Color" settings to "Display" settings. As a result ...
If it is flatpak install then Firefox profile should be located here:
whereas "normally" it resides here:
at least this is the case on my 5.7.1 elementary OS
In Firefox, if I select
Help>More Troubleshooting Information>about:profiles
I can open profile directory (but in my case it is not ...