Guidance on adding a plugin to install local firefox + arkenfox user.js

### Is your feature request related to a problem? Please describe

I'd like to have a project local Firefox profile, with a optional customized user.js from Arkenfox.

I know there's https://github.com/vincentvanbush/asdf-firefox but installing it using `asdf direnv local` fails:

```
asdf direnv local firefox latest
▶ asdf plugin-add firefox # ... plugin firefox not found in repository
✔️
▶ asdf install firefox latest # ... No such plugin: firefox
❗️ Failed with status 1
```

This is just about the extent of what I know about the problem, which I'm ready to solve if someone can point me in the right direction along the way. Where do I start?

### Describe the proposed solution

1. Add a `firefox` plugin that can be used with `asdf direnv local`
1. Add a `arkenfox` plugin that can be used with `asdf direnv local` and injects the requested version of Arkenfox in the currently configured firefox profile
1. In 2. protect against existing user customization overrides

### Describe similar `asdf` features and why they are not sufficient

- There is to my knowlege no firefox plugin that works out of the box with `asdf direnv local`.
- https://github.com/vincentvanbush/asdf-firefox does not verify any checksum or signature.

### Describe other workarounds you've considered

I've tried adding a user.js directly in '$HOME/.mozilla/firefox/'
I don't like messing with this HOME directory, and the profile names look absolutely horrible.
I'd rather have a local .firefox directory in a project-local directory, with a user.js file that symlinks
to a project.local Arkenfox user.js.

The issue names the asdf direnv local, asdf plugin-add, and asdf install entry points, but no repository files or tests. Start by comparing existing community plugin conventions and the linked asdf-firefox project; done would mean project-local Firefox installation, optional Arkenfox user.js injection, protection from customization overrides, and checksum or signature verification.

Written by the indexing model from the issue text.