Consider self-distributing the Firefox add-on

Context: https://github.com/DIYgod/RSSHub-Radar/pull/635#issuecomment-1100721069

#### TL;DR
> You really should read the context.

Add-ons distributed on AMO (https://addons.mozilla.org) are unable to use `eval()`, making remote rules updating and local rules modifications impossible. However, a self-distributed add-on faces much looser reviews so that's possible to enable both features. Here is a [toturial](https://extensionworkshop.com/documentation/publish/self-distribution/) on how to self-distributing Firefox add-ons.

You really should not read this

In fact, no reviews, but automatic checks and approval.


Shhh...

#### An effective example
[rsshub_radar-1.7.1-fx.zip](https://github.com/DIYgod/RSSHub-Radar/files/8500722/rsshub_radar-1.7.1-fx.zip) (already signed by AMO so it can be installed easily, change the ext name to `xpi` if needed)
Modifications on local rules are no longer ignored, making it possible to update rules by yourself, or debug your own rules before summiting a new RSSHub route, cheers!
As for remote rules, it is still disabled. But you must be a **magical** girl who loved a [**cyber chef**](https://gchq.github.io/CyberChef/), right?
```
P4xF1rYWBS+cSclbXFWDY4+ady5FUSK8PuHFYOM=bv7+p8J1poRKB9vcpW+cUWK1PzwySFwadNSlUx+1PugmpzPKavL+s+KAPr+mxrSJ5o5eovMApSPKYOvcB+n7XyxW
```

##### Disclaimer
- My self-distributed add-on comes with absolutely no warranty.
- So do remote rules. Technically speaking, enabling remote rules is allowing remote code execution (RCE), so **you are the only one responsible for your action**.
- The add-on contains some code from a drafting PR. I was just too lazy to remove it.
- The add-on contains some useless changes. I was just too lazy to remove them.
- You can audit the source code yourself: https://github.com/Rongronggg9/RSSHub-Radar/tree/ff

#### Related issues / PRs
Shouldn't #667 be enough? Probably not. Radar rules are now required for any new RSSHub routes. No reason to force a Firefox user to install Chromium or modify the code of RSSHub-Radar (then build and install on Firefox) before they can debug their radar rules, isn't it? What's more, a sideloaded add-on will be uninstalled automatically at quit (only Nightly, Beta, or ESR may escape if configured - but only [<20%](https://en.wikipedia.org/wiki/Firefox_version_history) Firefox users use these three channels), so that's also an unacceptable solution for ordinary Firefox users.

Here's a contributor who had to waste a lot of time before realizing RSSHub-Radar is almost completely broken on Firefox (https://github.com/DIYgod/RSSHub/pull/9537).

Other related issues: #687 #660 #408 #460 #675

Start by reading the context in PR #635 and the related issues #667, #687, #660, #408, #460, and #675. Compare the linked signed add-on archive with the source on the `ff` branch, then determine the Firefox self-distribution and update requirements; done should include an agreed distribution approach and validation for local and remote rules.

Written by the indexing model from the issue text.