i've just tried it on this forum and i couldn't upload an avatar here.
there is no error message.
user name > avatar (from the drop down)
i click the 'choose file' button
when the window opens for you to select a file opens, i can navigate to my folder.
as soon as i select a file, the form on xenForo disappears and no file appears in the dialogue box in windows explorer!
hope this makes sense, if not i'll elaborate.
(safari right out of the box, i just use it for testing)