I looked at this just a few months as I have a few extensions with some very me-specific stuff that I don't really need/want to distribute – it's just not going to be useful for anyone except me. I couldn't find a good way to permanently install an unsigned or self-signed extension.

You can temporarily add unsigned extensions in about:debugging, but those are lost on restarts, which is pretty annoying. I used this for a while until I got fed up and tried to find a better way.

"Unbranded" Firefox builds allow adding unsigned extensions, but then I need to either 1) compile my own Firefox, or 2) Use "Firefox Developer Edition", which is mostly just the same as regular Firefox but based on beta versions (I'd rather just use release versions). Neither really appeals to me.

So my solution now is to just create "unlisted" extensions and sign them with the web-ext CLI. It works and it's not entirely horrible, but it's a lot more hassle than I'd like.

And the requirement for extensions to be signed is fine; I have no problem with that. But it should allow adding my own signing key. Or something.

I kind of get why Mozilla is so restrictive about this; with banking and credit card stuff and whatnot all being browser-based, adding an extension is basically giving the keys to the castle. I can see some support scammer instructing someone to add some malicious signing key. But there does need to be some limit to how much we protect people from themselves, because at some point you just start making life hard for regular users.

You must be using either the Developer Edition, ESR, nightly or some unbranded version. Vanilla Firefox doesn’t allow to install unsigned extensions permanently.