28/08/2019: Why does GTK3 Firefox require GTK2?
Firefox only supports Linux builds with GTK3 for some time now, however GTK2 is still a mandatory dependency. It turns out that GTK2 is required for NPAPI plugins and the only plugin which is still in use is the Flash Plugin!
You cannot disable NPAPI plugin support which means that Firefox has a dependency on GTK2 solely for Flash. The sole reason that KISS ships GTK2 is for Firefox, Flash will die some time in 2020 (possibly 2021) so until then we're stuck with GTK2.
As long as NPAPI plugins are supported, build dependency on gtk2 cannot be dropped.
NPAPI is currently only around to support Flash (bug 1269807). Per https://developer.mozilla.org/en-US/docs/Plugins/Roadmap, Firefox's Flash support (and thus NPAPI support) will be removed sometime in 2020.