1. Safari -> File menu -> Export bookmarks
2. Transfer the resulting .html file to the PC
3. PC Firefox -> Bookmarks Menu -> Manage Bookmarks
4. PC Firefox Manage Bookmarks Window -> File -> Import and select the file from step 2.
In Tiger just export the Bookmarks to a html file with File menu -> Export Bookmarks...
In Panther it's just as easy, but you need to enable the Debug menu, because the Export Bookmarks is located in the Debug menu (at least in 1.2, haven't tried 1.3). To enable Debug menu, either use a tool like TinkerTool ot open Terminal and type/paste: defaults write com.apple.Safari IncludeDebugMenu 1
Edit: Then follow mkrishnan's advice to import into Firefox...
You can sync your bookmarks between different Macs using .Mac, and you can access your .Mac bookmarks online from any Mac or PC, but to truly sync across platforms... now that sounds difficult...