I am sure this is possible with an elaborate software design, however at the moment there is no option or add on that open all mail to or email url with gmail.

There are some other enhancements that I can recommend:

1. change the default mailto: in the Applications menu
2. There are some gmail hot key addons you can add to your toolbar https://addons.mozilla.org/en-US/fire.../?q=gmail&appver=48.0&platform=mac

You can also try the share this page, airplane icon with these video instructions: http://screencast.com/t/TO6CM7DxwK

Maybe I wasn't clear or I don't understand - could go either way! When I am browsing, say on Groupon, and I see something interesting that I would like to send to a friend how can I get Firefox to use my gmail account. I'm old, but I believe I have been able to do this in the past.

The old way, which may or may not still work in Windows 10, was to use the Options page as described in the following article:

Change the program used to open email links

Could you test whether that still works, or whether the built-in method is obsolete?

Oh, the second half of the puzzle is the Email Link command/button.

You can access it from the classic menu bar, File menu. If you aren't displaying the menu bar, you can activate it temporarily by tapping either the Alt or F10 key.

There also is an optional button available to add to the toolbar or graphical menu in Customize -- it has an envelope icon. See: Customize Firefox controls, buttons and toolbars.