Overview:
Sometimes there is a disconnect when you right click on a URL Shortcut in the filesystem and notice that the Send to: Mail Recipient context menu entry will not work.
The following procedure will easily correct that problem for Mozilla Thunderbird and other mainstream email clients, letting you have this important functionality back.
Just follow these simple steps:
1) Open File Explorer and erase the current entry in the directory path field.
2) Then paste this in the path field: %appdata%\Microsoft\Windows\SendTo
3) The folder in the path will pop up. Mail Recipient will be clearly listed in the Folder. You can move it somewhere else or delete it.
4) Create a Desktop Shortcut for Thunderbird, or copy the default one already created.
5) Paste that Shortcut directly into the SendTo folder.
6) You can close all unneeded open windows at this point.
7) Click Right on an appropriate item and Send to -> Mozilla Thunderbird will be listed there with it's icon.
8) Click on that entry and Thunderbird will open the Message Pane with your attachment.
Again, you can also use this same procedure for other email clients, except UWP Mail Apps.
Just substitute the appropriate email client shortcut as described in the procedure above.
That's it.