So I understand that you want to replace stock Gtk dialogs (which don't work for you) with Trinity ones? In this case kgtk is the way to go.

I don't normally use it so I can't say much, but AFAICT both kgtk PKGBUILDS in the AUR are for KDE, not Trinity, and kgtk is not oficially packaged for Arch (why?). The best way would be to install it from source then, but I'm not sure if it works with modern Firefox.

Sorry if I can't be of more help to you.

--
Philippe