Edit: I figured it out. The problem wasn’t permissions or Firefox itself, but my launcher. I’m using OLauncher, 'cause I rather like it’s text-lookup interface, but as it doesn’t make use of a traditional “home screen” Firefox won’t allow me to create an entry there.

Apparently, it’s a much-desired feature request

I think it may have been renamed to “Install” in previous versions, but neither appear anymore, and I don’t see any permissions denied so I’m assuming that can’t be it.