[bug 855671] Fix Windows version parsing for Firefox 3.6

Bug 855671 covers two problems with UserAgent parsing. This fixes
one of them and adds a test for the second, but I wasn't able
to reproduce the second, so I'm puzzled.

I'm going to land this and then see if the second one (TypeError: cannot concatenate 'str' and 'NoneType' objects) happens again.

r?