If you don't want to wait until Mozilla provides an official fix for Android, you can either:
Download: https://storage.googleapis.com/moz-fx-normandy-prod-addons/extensions/hotfix-update-xpi-intermediate%40mozilla.com-1.0.2-signed.xpi
(I got it from https://normandy.cdn.mozilla.net/api/v1/recipe/, search for ''fix bug 1548973'' on the page and you'll see it. Also from Twitter: https://twitter.com/firefox/status/1124797980450656256.
Go to about:config, set xpinstall.signatures.required
to false
. Note: This workaround should only be used temporarily, as it disables a security feature. Please remember to set the xpinstall.signatures.requiredentry back to true again once Mozilla provides an official fix.