After doing some searching, here's the deal.
Firefox stores all of the history and form submit history in the .dat files located in your firefox profile directory. The file you are looking for is formhistory.dat.
The problem with this is that the file is a binary file formatted by something called Mork, which nobody has ever heard of because it's simply that bad.
Firefox 3.0 will use a different mechanism to store this information, so Mork is just hanging around until they finish the new release.
There's more information, and a utility to convert the history.dat file (but not the form history) at this link:
There is a python script which I have not tested that can be used to convert any of the .dat files to something like xml:
For Internet Explorer, form auto-complete information is stored in the following registry key, but is encrypted:
Hopefully that answers your question =)