Both Firefox and Thunderbird contain configuration files that can be edited. Editing these configuration files allows you some flexibility in making changes to both the program itself and its appearance. Hacking these files is relatively simple, but bewarethings can sometimes go awry, so before you delve into this area, read this appendix carefully, and be aware that sometimes things might go completely haywire if, for example, you mistype a line. Rememberyou have been sufficiently warned. Have fun, but be careful!