Screenscraping w/ Mozilla or Firefox

I must be using the wrong search words, but I can't find any information on a 
way to script Mozilla or Firefox.  There are various tools that use IE but I 
was kinda hoping to run a process from a Linux box using Firefox.

I've read about XPCOM but that sounds more like a mechanism for extending the 
browsers than it does a way to script them.

There was a little bit of information about PyXPCOM but that doesn't look as 
though it's been maintained and it doesn't seem that with an interface 
designed for this purpose it would be necessary to build Firefox or Mozilla 
from source.

Any pointers or keywords would be helpful.  Thank you.