I have seen the same behavior in FF2+ with PKZip - not sure why, but if I click on the inactive (grayed out) OK, after a millisecond it becomes available, and then I can open the Attachment.
I don't know how to change the OK/Cancel default, and I wouldn't for security reasons.

[/acronym] [acronym title="What up Dude?"]
[/acronym] UTC -7ąDS


