Simple: They are queued up inside Firefox until one of the other 7 sends
back its reply, then the first one in line gets sent and so on.
This is actually a general part of the HTTP/1.1 standard
(http://www.rfc-editor.org/rfc/rfc2616.txt) in the last paragraph of
section 8.1.4, except that standard says the limit should be only 2
requests pr. server pr. browser (for all tabs and windows added
together).
As a writer of ajax or other web applications, you can find many other
important rules in that standard, such as rules about what things are
not allowed to happen as a result of a GET request (but are allowed to
happen from a POST request).