Seamonkey browser not honouring Gtk font settings

I am having some minor trouble with SeaMonkey 2.49.1 on Linux (Manjaro 17.1.1 KDE). SeaMonkey isn't affected by my Gtk font settings. Instead it uses a really thin and barely legible font. It is the only Gtk (both Gtk2 and Gtk3) program on my system affected by this, and it affects both the official Gtk 3 build, and a custom Gtk 2 build I finished compiling. For comparison (top to bottom: Gtk2, Qt5, Gtk3, SeaMonkey): https://ibb.co/j6SODb " class="img-fluid rounded" style="max-width: 100%; height: auto; margin: 10px 0;" loading="lazy">