The bare minimum to keep the old XUL-based about:config version available in Firefox 87+

Mostly copied from https://github.com/xiaoxiaoflood/firefox-scripts, all credit goes to them.

All credit goes to @aminomancer for the idea and the code for registering a proper about: page!

1. download and extract the master.zip
2. copy the files/folders from installdir into your Firefox installation folder and the chrome folder from profile into your profile directory
3. load about:support and click the button "Clear startup cache..." (not sure if this is really necessary)
4. restart Firefox
5. you can now access the old about:config via chrome://userchromejs/content/aboutconfig/config.xhtml

Optionally you can also let the script register an about:cfg page. To do that, copy/move aboutcfg.jsm into the profile/chrome/utils/ folder and uncomment the 2nd-to-last line in the _autoconfig.cfg file.
Now you can access the old about:config page at about:cfg !

• autoconfig.js tells Firefox to load the autoconfig file _autoconfig.cfg
  • _autoconfig.cfg tells Firefox to load the chrome.manifest from the chrome directory in the profile folder
    • chrome.manifest registers a chrome:// namespace userchromejs

Mozilla removed the old about:config in https://hg.mozilla.org/mozilla-central/rev/2e2f7a1fd4fa

see the above link for the original files:

• toolkit/components/viewconfig/content/config.js
• toolkit/components/viewconfig/content/config.xhtml
• toolkit/locales/en-US/toolkit/about/aboutConfig.ftl
• toolkit/themes/shared/config.css

The only modifications are:

1. hardcoding all the localization stuff from aboutConfig.ftl into config.js and config.xhtml
2. remove the unnecessary telemetry stuff from config.js