Ok, as for my first problem with mogrify, it turns out I'm just a fool. The libraries I needed apparently weren't part of GTK. I had to download pango and cairo (as well as a pile of other libraries) before it started to work.
As for firefox 13, my system is 64-bit, and the file is 32-bit. After a little searching on here, I've found this link
detailing how to set up 64-bit slackware to run 32-bit programs.
Thanks for the help volkerdi.