“I always click on a link to download a file and wait for a firefox window to appear. This window contains two options: "Open with " and "Save to Disk", and three buttons: "Browse...", "OK" and "Cancel".”

“The problem is that "OK" button is playing up. Very often it's inactive. Sometimes clicking on the "Browse..." button or shuffling between "Open with" and "Save to Disk" or closing the window and trying to click on the link again magically makes it active. Sometimes it remains dead forever, though. See the picture attached.”

For me this problem went away when the window containing the buttons is not a pop-up window.

[http://support.mozilla.com/tiki-view_forum_thread.php?locale=en-US&comments_parentId=4507&forumId=1]