"Just now I only had TB 32 bit installed.
I installed TB 64 bit and rebooted.
Now I had both TB 32 bit and TB 64 bit installed.
With TB 32 bit set as the default mail program, send to mail recipient worked fine.
With TB 32 bit set as the default mail program, send to mail recipient did not work.
I uninstalled TB 32 bit and rebooted.
I uninstalled TB 64 bit and rebooted.
I reinstalled TB 64 bit and kept the option to Use Thunderbird as my default email client (which I always leave set every time I install TB).
At this point the windows settings showed TB as the default mail client and TB showed itself as the default mail client, but send to mail
recipient did not work.
I uninstalled TB 64 bit and rebooted.
I installed TB 32 bit and rebooted.
At this point send to mail client worked just fine."