Fix API signing of Mozilla Firefox Extension for Manifest V2 and/or MV3

[Jaifroid]

This issue supersedes #314. I wrote there that there is not much call for nightly signed versions, and that we haven't been able to sign these programmatically for a number of months or even years. However, if we switch to Manifest V3 for the Firefox extension, it will be important to be able to sign the V2 legacy package that we should continue to build.

The reason this is necessary, is because unlike Chromium, temporarily loaded unsigned extensions are erased from Firefox browsers once the user exits. So we will need a package that the user can install without being erased, i.e. a properly signed package.

The NPM script for signing is still regularly updated, so it seems the API is still fine. It's possible that our API keys need to be refreshed.

See https://github.com/mozilla/web-ext.