In Fx3b5 Tools > Options > Applications, where does it get the Action
value for mailto from?

It's either getting "Use Windows Mail (default)" from the wrong place
or wrongly suggests to me "Windows Mail" is my default mail client.
Perhaps instead of "Use Windows Mail (default)" it [sh|c]ould say
"Windows (default)" or perhaps ""

In Tools > Options > Applications, it says:

Content Type Action
mailto Use Windows Mail (default)

but "Windows Mail" is /not/ my mail client nor is it the default.
Typing into the Location Bar opens my default
mail client, and is also the default mail client according to "Default
Programs > Set Default Programs", the Start menu, and under Custom in
"Set Program Access and Computer Defaults" "Use my current e-mail
program is selected and "Windows Mail" is /not/ selected.