A browser and its bookmarks are my personal window to the world, I wouldn't enable a feature like sync for love or money. What a crazy idea: hey ad company, here is everything I bookmarked that I wouldn't dare even mention to my closest confidants
Firefox Sync encrypts all data client-side; Mozilla has no way to read it. Chrome also supports client-side encryption as an optional feature using an additional passphrase.
FWIW, although this isn't on by default, Chrome can be configured to encrypt all sync data, either with your Google password (which Google hopefully doesn't store) or a separate, offline, password.
