From what I can tell this is not possible.
I'd recommend getting in the habit of closing the tab then stopping debugging.
Alternatively, you could select the Local IIS option
Right Click the Project > Web Tab > Servers Section > Select the 'Use Local IIS Web Server' option.
If you have IIS installed it will create a virtual directory for you. You can rebuild and just navigate to your local url. Debug if needed or even attach to the w3wp.exe if you want.
This mode is very handy when you're just doing layout stuff as well and you don't care about the code but just want to update markup etc. and refresh quickly.
Related:
http://stackoverflow.com/questions/247908/how-to-prevent-tab-hell-with-visual-studio-and-firefox-debugging
http://stackoverflow.com/questions/1171475/when-debugging-an-asp-net-application-how-to-end-debug-session-when-firefox-clos