There is a technique for adding a new mail service to that drop-down list, but you need to find out the URL for composing a new message in that service. This thread describes the method for Hotmail / Windows Live Mail, but obviously you would need to replace that URL with the one for your mail service: hotmail | Firefox Support Forum | Firefox Help.

Solved!

I was mistaken it is only partially working. It navigates to the iCloud mail application but not the compose page. Here is the script/string I entered:

javascript:navigator.registerProtocolHandler('mailto','https://www.icloud.com/#mail/secure/start?action=message/en-us/#compose&to=mailto:%s", iCloud Mail')

What have I done wrong? I've been playing with variations but this script get me the best result.

Hi BobS, I'm trying to think of a way to figure out the link to generate an email message so we can learn the right URL. Does the address book in your iCloud mail let you click a link to create a new message to a contact? Perhaps that would be the right link.

That was a good suggestion. I tried it with the resulting URL being https://www.icloud.com/mail/message/en-us/#compose?to=xyz@fictitious.com I have changed the recipient in the example. I then substituted the recipient address with %s (from the hotmail example) The result is a blank page and the address is corrupted with '%3A' before the name '%40' inplace of @ symbol

I will inquire with Apple support & see if I can glean more info. If successful i'll post it here

What would you refer to the characters representing the destination address after "to=" as? I call it a wild card is that correct?

I tried the apple support form but no response. I'm optimistic that I'll get a result from Apple; there is an outstanding ticket on the issue.

I'm not sure of the formal name for the %s parameter that Firefox replaces with the URL. Placeholder? Variable?

Background information: Web-based protocol handlers - MDN

I heard back from Apple. They were unable to get the compose page to open in iCloud mail using Firefox as the browser. I was informed they are willing to work with Mozilla developers to resolve the issue.

How can I contact a Mozilla developer in order to help?

I think some Mozilla developers work actively with Apple on various issues, but they might not be familiar with issues using Apple services from the Windows platform. If you don't mind registering for the Bugzilla site, you could search there for this issue and, if you don't find it, submit a new bug.

https://bugzilla.mozilla.org/