Nice one - bravo and reps - Firefox should have made this a simple option to vary these by selecting connection type or somesuch (dial-ups days are surely numbered, although it is still no doubt wise to default in that direction!)
This has been tested on 1.5-3mbit DSL connections with moderate to significant improvement. (Not intended for dial-up) Remember to save your settings if you want to reset.
Type "about:config" in the adress bar.
Find these options, double click each one and change to below values:
Close Firefox and reopen. Some people told that these settings may hammer some servers. Changing the "network.http.pipelining.maxrequests" to max of 8 will probably solve the problem.
(reduced to 8 from 16)
oops ! Sorry guys. I should of checked better before posting.