Well, easier is just to :
⋅ first sync’ or backup your firefox profile ;
⋅ then remove Firefox as a snap
⋅ install Firefox from autonomous archive provided by Mozilla ( which auto-updates ) ;
⋅ or install Firefox from MozillaTeam ppa ( then updates through your apt manager ).
Those ( archive or .deb package ) will follow your gtk theming and fully work as expected.