My buddy and I built an extension called Hush that allows you to save them behind a password of your choosing. They're encrypted using SHA-256/AES, and you can export/import them to your liking (in case someone accidentally uninstalls the extension). It only operates in private browsing for security reasons.

Check it out and let us know what you think!

Chrome link: https://chrome.google.com/webstore/detail/hush-private-bookmarking/hjmoaenjknbdehbiaeeijcppnljflkff

Firefox link: https://addons.mozilla.org/en-US/firefox/addon/hush-private-bookmarking/

More information: http://teameuler.com/hush/