Email link tries to start new instance of Thunderbird Portable

I use Portable Apps on my hard drive (to keep from getting tangled in the registry so I can be portable if I need to). My default email program is set to Thunderbird Portable in both Windows 10 and FF Portable. But if TB Portable is running (as it almost always is) and I click on an email link on a webpage, instead of just opening a composition window, I get: "Another instance of Thunderbird is already running. Please close other instances of Thunderbird before launching Thunderbird, Portable Edition." Is there a setting I need to fix somehow?