What happens exactly ? Please provide more Information. Is this add-on on Firefox Add-ons site ? (Cuz I couldn't find it there !) Does it show you an Error or something ? I just delved into this website : http://www.speedbit.com/dap-help/browser-integration/ It looks like there's no official extension being referred to.

More info please ?

Starting with Firefox version 48, the add-on signing requirement will be enforced with no override in Release and Beta versions of Firefox. See the MozillaWiki article, Add-ons/Extension Signing for more information.