This WebExtension has been retired. It is really beyond the scope of what a WebExtension can do; APIs used by HTitle are gone and workarounds aren't really there. Specifically, the lack of window controls would always have remained an unsolved problem. I could have continued to pile hacks upon bodges, but in practice this project would never really have become particularly stable or reliable.
Mozilla are working on doing this properly — it's bug 1283299 if you want an idea of progress. Ultimately it makes far more sense simply to wait for this to be released.
Many thanks to everyone who commented and submitted code.
Hide Title Bar
An addon for Firefox, compatible with GTK 3.
Unsigned versions can only be installed on developer editions of Firefox, or temporarily through
Releases are at https://addons.mozilla.org/en-US/firefox/addon/hide-title-bar/.
The native script can be installed by hand or through a Debian package; these are both accessed from within the add-on.
An Arch Linux package for the native script exists at https://aur.archlinux.org/pkgbase/hide-ff-title-bar-git/. This package >was produced by a third party so I cannot accept responsibility for its contents.