As far as I know, there is no such thing as "
setting the homepage" that works in Firefox. People have been looking for years on a solution that is cross browser compatible, but in the Mozilla forums, nobody has come up with a solution that I have found yet. There are only scripts that will allow you to add your site to your favorites or just bookmark it, such as this one:
Dynamic Drive DHTML Scripts- Bookmark Site script
In all honesty, most users use IE 6 or 7 anyway. If the "
add this site as homepage" is that important to you, then use the code you already have and leave the Firefox Guru's to do it manually. I do all that crap manually anyway, even if there is a "add this" link on the page, as I am so used to it. (Ha!)