[I edited my answer, because now I read that new system is claimed to be end-to-end secure as well]<p><a href="https:&#x2F;&#x2F;support.mozilla.org&#x2F;en-US&#x2F;kb&#x2F;sync-your-firefox-bookmarks-history-passwords-and-" rel="nofollow">https:&#x2F;&#x2F;support.mozilla.org&#x2F;en-US&#x2F;kb&#x2F;sync-your-firefox-bookm...</a><p>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]).<p>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&#x27;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.<p>[1] <a href="http:&#x2F;&#x2F;www.cnet.com&#x2F;news&#x2F;mozilla-adopts-plain-vanilla-password-sign-in-for-firefox-sync&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.cnet.com&#x2F;news&#x2F;mozilla-adopts-plain-vanilla-passwo...</a><p>[2] <a href="https:&#x2F;&#x2F;support.mozilla.org&#x2F;en-US&#x2F;kb&#x2F;firefox-sync-upgrade-frequently-asked-questions" rel="nofollow">https:&#x2F;&#x2F;support.mozilla.org&#x2F;en-US&#x2F;kb&#x2F;firefox-sync-upgrade-fr...</a>