Custom `geo`, `locale`, and update channels

There are instances where you may need to change your local build's locale, geo, and update channel (such as changes to the visibility of Discovery Stream on a per-geo/locale basis in ActivityStream.sys.mjs).

Changing update channel

• Change app.update.channel to desired value (eg: release) by editing LOCAL_BUILD/Contents/Resources/defaults/pref/channel-prefs.js. (NOTE: Changing pref app.update.channel from about:config seems to have no effect!)

Changing geo

• Set browser.search.region to desired geo (eg CA)

Changing locale

Note: These prefs are only configurable on a nightly or local build.

• Toggle extensions.langpacks.signatures.required to false
• Toggle xpinstall.signatures.required to false
• Toggle intl.multilingual.downloadEnabled to true
• Toggle intl.multilingual.enabled to true
• For Mac and Linux builds, open the langpack for target locale in your local build (eg firefox-70.0a1.en-CA.langpack.xpi if you want an en-CA locale).
• For Windows, use https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central-l10n/
• In about:preferences click "Set Alternatives" under "Language", move desired locale to the top position, click OK, click "Apply And Restart"