I just installed the latest version of Firefox 3 on a fresh Leopard partition. The copy of Firefox on my Windows desktop has all my bookmarks and saved passwords. I originally thought of only transferring the bookmarks to the Firefox on Mac, but I experimented with copying the entire profile folder over. Nothing to lose!
I copied only the contents of the original profile .default folder, emptying and replacing the Firefox profile folder on my Mac. Opening Firefox on my Mac immediately gave me the familiar state of the Firefox on my Windows, exactly as I had it (before I transferred). All my saved passwords, searches, and history are also there as well!
Will report any issues but everything works great so far.