You can work around this by running Firefox Developer, which is able to disable Mozilla's signing requirements. That edition of the browser actually respects xpinstall.signatures.required, so just toggle that, zip your extension, rename to .xpi, and install. It'll warn you once, proceed, and persist through browser restarts just like any other extension.

Obviously that's no help if you want to distribute the extension (we have to jump through hoops at our organization to push out a manifest) but for personal use I find it to be a good workaround.