Firefox is way behind Chrome on this front. You can't permanently install a private extension in the release version of Firefox, unless you upload the source code to their servers for signing, and the unlisted extension complies with their developer policy.
Mozilla's stance on extension signing is hostile to users, there are several extensions that got remotely blocked for loading code from Google Translate [1]. This is software that you privately install for your own use, and Mozilla blocks it remotely in your browser.
They've also rolled out the new version of Firefox for Android which only supports a couple of extensions that they have whitelisted. I can no longer use the extensions I have developed and published on Firefox Addons, and I can only imagine the grief it is causing for less technical users that have suddenly lost the productivity tools that they've relied on.
They've also disabled about:config in the new version of Firefox for Android, you can no longer configure the browser that way.
[1] https://www.jeremiahlee.com/posts/page-translator-is-dead/