How to get External Client option working in 5.1.0c?

[namecaps]

Dear All

As subject says - I'm trying to get external client option to work when I click on compose or reply to button. How to achieve that? I've already set options in Admin menu as well as My Account (to External Client) but it does not work (still internal editor opens up). I'm struggling with it since morning and haven't had any luck.

Also how to setup From to be default to personal settings? (it's do_not_reply@example.com right now and you have to open dropdown From menu and select the account).

Regards

EDIT: When I enter mailto:"someone@somewhere.com" in start/run under windows thunderbird opens (that if you'd ask about default email client).
I tried in Fx and IE, no luck.