I went and busted my firefox somehow so that it ignores Theme changes in Gnome Tweak Tool. All other windows obey, but Firefox is stuck.
Any ideas what I did? I did not attempt any mucking about with FireFox’s horrible, hidden config files in .config. I’ve just been using Gnome Tweak Tool and the Firefox Addons/Extensions.
No matter what I do, Firefox’s titlebar looks like this:
All other windows conform to whatever theme I pick: