This updates frequently since it is basically Firefox beta (though in practice i never had any issue). Note that this has telemetry enabled by default so you may want to disable it via the options.

Unbranded stable: https://wiki.mozilla.org/Add-ons/Extension_Signing#Unbranded...

The unbranded stable isn't updated automatically so you'll need to check for new versions yourself (shouldn't be too hard to automate it with a python script or whatever that runs via cronjob or windows task scheduler and notifies you for new versions).