sideload addons on android

README.md

• Install the extension you want in desktop firefox, copy the path the extension(.xpi) file you want to install and copy the content relevent to the extension from exetensions.json(in firefox profile dir)
• Change all the paths in that section to android paths(usually /data/data/org.mozilla.fenix/files/mozilla/<random string>.profile/extensions/<extension id>.xpi)
• Edit the extensions.json in the android profile directory and include the section you modified above in addons array.
• Copy the xpi file to /data/data/org.mozilla.fenix/files/mozilla/<random string>.profile/extensions/<extension id>.xpi
• Enable adb debugging, enable remote debugging via usb on firefox android
• Goto about:debugging on desktop firefox and connect to firefox on android, then select Main Process -> inspect and get the console
• In the console run the following code, make sure to replace the relevent sections with correct paths, and keep a tab loaded with a webpage in fenix

Cu.import('resource://gre/modules/addons/XPIInstall.jsm')
Cu.import('resource://gre/modules/addons/XPIProvider.jsm')
Cu.import('resource://gre/modules/addons/XPIDatabase.jsm')

var nsIFile = Components.Constructor( "@mozilla.org/file/local;1", "nsIFile", "initWithPath" );
var xpiFile = new nsIFile('<full path to xpi>');

var addon = await XPIInstall.loadManifestFromFile(xpiFile, XPIInternal.XPIStates.getLocation('app-profile'));
addon.appDisabled = false;
XPIInstall._activateAddon(addon)

• This will install and activate the addon, to make sure it's preserved accross restarts, kill fenix right after the above step and start it again.