Comment 3 for bug 257510

It's a bug because it's not clear _why_ the button is disabled. And this obscurity even makes it seem like a different bug.

If both the buttons (Cancel too) were disabled then it would be more obvious. Tooltips on the disabled buttons would also be explanatory. In fact, if a button is going to be disabled then all the UI elements should be disabled. It's just as bad to click the "Open with" radio choice by accident without knowing as to click OK by accident.