Safari 1.x (Panther) or 2.x (Tiger)?
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... 😉