Good advice all around from ctpkrf.
Overall, I would encourage you to (and I know this is a pain, but) re-do all your old FP sites in something that produces at least marginally cleaner, more up-to-date code -- but it really depends on what sort of sites these are.
iWeb produces very nice, functional web sites for mostly personal-type sharing (blogs, picture galleries, that sort of thing) using templates. It's a hand-coder's worst nightmare, but its kinda the FP of Macintosh.
For something beyond the basics, you might look at Rapidweaver, Sandvox, or Freeway -- also template based, but much more flexible and write better code.
If you know enough HTML or XHTML/CSS et al to understand what is going on with your WYSIWYG type sites and can do your own tinkering, Seamonkey is an excellent choice, and you may want to consider moving up into the big-league apps like Dreamweaver and Flux (and there may be some other options along the way I've overlooked).
Lots of choices, but ultimately -- it's probably best that you re-do these sites in more modern code at some point. You can always continue to use FP via a Windows virtualising environment like Parallels (et al) if you don't want to tackle this chore right away.
