hidden settings are hidden for a reason
there are pro's & cons to altering the values
the default settings (usually) are ideal for the average user

I'd advise you read up on exactly what the settings do before fiddling
as it can impact on your system, and the sites you connect to
http://www.tweakguides.com/Firefox_9.html

That said, the tweaks above certainly improve speeds if you have a broadband connection