Note that this doesn’t currently work in Firefox 1.5
Specifically, serialised as RDF/XML.
Early last year, Jamie Zawinski asked about how to extract information from Firefox’s history.dat file and ended up having to write a Perl script to do it.
He actually got further than most – most people who try to access the data not via Firefox itself just give up. This is because Firefox’s history is stored in a Mork file, which is an incredibly over-complicated and under-documented proprietary file format. You’ll learn more about it by reading the comments on Jamie’s post that you would by reading the docs.
Mork is used as a file format all over Mozilla, in both Firefox and Thunderbird in various points, but there are plans to replace it with SQLite in Mozilla 2.0 (see Unified Storage on the official Mozilla wiki) possibly with some interfaces made public via RDF (but I’m not 100% sure on this).