I use Bookdog. I keep my bookmarks synced across Safari, Firefox, and Camino. I think it works with other browsers too. They've recently introduced the successor to Bookdog, BookMacster, but I haven't played with it that much because Bookdog does what I want it to...and what you want it to do also.
You also might want to google xmarks (formerly foxmarks). I haven't used it in a while, but for a short time I was syncing my Safari bookmarks with xmarks, and then xmarks (like MobileMe) would keep all my Firefox bookmarks in sync across my macs.