Cannot add Zoho Mail as default webservices in Firefox Quantum

Cannot add Zoho Mail as default webservices in Firefox Quantum

How can you add Zoho Mail as the default mail client in Firefox. There used to be an option in the settings where you could choose 'Zmail' as your default client but that has been removed. The firefox support page suggests adding an addon to allow to you add other webservices but the addons arn't there or are out of date. It's sooo frustrating.
See link to support page:
https://support.mozilla.org/en-US/kb/change-program-used-open-email-links

Any ideas? ;0)