Brave and i believe a few chromium browsers have the ability to switch to Qt, GTK or their own graphics manager as they please, i believe that should also be the case for firefox as GTK app on Qt looks weird, especially on non-KDE DE's like LXQt. I know that might break a few themes but you can just use like Qt themes or GTK themes on Linux, setting Dark or Light instead of auto already makes a weird GTK + Windows Firefox proprietary hybrid theme.