Keep in mind Firefox is actively attacked every day and has high exposure to unknown entities and sites. I doubt many people are attacking syncthing-gtk very often and it doesn’t connect to random websites all day.
Unless you are guarding against a targeted attack on you or the Syncthing community I would say Syncthing-gtk would be “safer” but this feels like a quality of life question.
Technicaly it is probably safer to live in a hermetic chamber in a reinforced bunker but that will make it hard to live my life.