[latin]Ave[/latin] ;) Stop fighting, create a wrapper script that sets the env. variable or adds the argument, put it in /usr/local/bin and make sure the application launcher doesn't hard-code the path. I do this kind of thing all the time with the Lancelot launcher (I'm still rocking a KDE4/Plasma4 desktop); sadly it appears no one migrated that launcher to KF5.
There may also be a possibility to create a custom .desktop entry in ~/.local/share/applications, and set the argument you want in there. Or you put it on your desktop. Lastly, have you checked if there's no way to toggle the setting via about:config or the equivalent of the "experiments" that Google Chrome has? (Or ... install the Firefox Developer Edition and use that, I've found it to be considerably faster ;))
