[I edited my answer, because now I read that new system is claimed to be end-to-end secure as well]

https://support.mozilla.org/en-US/kb/sync-your-firefox-bookm...

In the old system your data was encrypted with a key that was only stored on your devices. Adding a new device meant that you had to do a kind of key exchange process (which was perceived as complicated[1]).

When Mozilla introduced the new system there was very little information on how the data was encrypted. I think the documentation only said that they used TLS (or something like that). But when reading their current documentation I see that it's not the case; they are apparently encrypting your data with a key derived from your password. So if you use a (cryptographically) strong password it should be secure[2]. Assuming that it works as documented of course.

[1] http://www.cnet.com/news/mozilla-adopts-plain-vanilla-passwo...

[2] https://support.mozilla.org/en-US/kb/firefox-sync-upgrade-fr...