Using "email link" feature. Regardless of how I set the email client to be Thunderbird, either in FF or in the OS (Win 7 64bit), TB starts up in the profile manager, and because TB is already running, I am unable to compose a message in the currently-used profile. I have 3 profiles defined, and I do not want to use either of the other 2. I observe that the OS believes that the command line for running TB (from HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Mozilla Thunderbird\Protocols\mailto\shell\open\command ) is "C:\pctools\Mozilla\Thunderbird\v38.4.0\thunderbird.exe" -osint -compose "%1" That is the correct path on my installation. If I manually run that command from a command window, for example: "C:\pctools\Mozilla\Thunderbird\v38.4.0\thunderbird.exe" -osint -compose or "C:\pctools\Mozilla\Thunderbird\v38.4.0\thunderbird.exe" -osint -compose "body='foobar'" then TB starts up with compose window as expected (including "foobar" in that case). I cannot, however, without doing some surgery on the registry, determine what argument FF is sending to the OS for the "%1" argument to TB. If I knew the latter, maybe I could fix this problem. Or, maybe someone can tell me how to fix the problem at user level? I recall that years ago I had a similar problem on an XP machine, but I am not sure how I fixed it. I think I may have copied an Outlook version of mapi.dll into the windows system directory. However, it appears that now TB and/or FF have their own version of mapi.dll (with a somewhat different name).