If it is not working out of the box I'd be interested in why, what's the name of your GTK theme?
It works out of the box indeed but IMHO this is not a user improvement but a regression because now users of the high contrast theme see all the issues with this Firefox mode. The main issues regarding this are: images are hidden often like the Facebook logo and it is difficult to differentiate part of a website because border are not respected.
A good example of this is the Firefox preferences where the highlight are not respected at all.
Could we revert this change or change the default Firefox behavior for this? As a visual-impaired person using the high contrast theme I don't want see site differently as others except if I request it.
The default behavior IMHO should be never. Usually low-user who want to improve contrast on website user color filters of the OS or screen magnifier or third-party module like the Dark Mode module that works pretty well.