If FF is opened, Email links will not open. Any fix?

If I have Firefox Nightly x64 opened and click on a URL link contained in a Thunderbird email, I receive a message warning that states that FF is running and need to close before I can open the selected URL link. See attached image.



I don't really want another FF session opened. What I would like is for the URL link I clicked on the email to open in a new tab (or on existing tab for all that matters).

Is that possible? Is there a setting I can toggle?

Thanks.

Yes; Thunderbird is my default email application. Did as you suggested but still have same issue.

Would like the selected URL link in email to open in a new TAB if FF is already running. As it is now, if I close FF from Task Manager (as suggested) or "Exit" FF normally; then when I select an email URL, FF opens. All other URL's selected on email open in a new Tab as well.

Basically, I have to "Exit" FF before I can view an email URL. Annoying if several tabs are already in use when I receive an email with a link I need to visit.