If you select Contrast Control Automatic, it turns on and off based on the high contrast mode setting on the system (on MacOS it's the "increase contrast" toggle).
In manual mode it's always on. I would like an option that syncs the manual on/off state with the systems on/off contrast setting, like the automatic mode, so I don't have to turn it on and off in Firefox separately.
I like to use the manual mode because I can change the colors and force the background to be white on all sites.