Lightning 2.6 is going to be the release you are stuck with for about 42 weeks, so its vitally important that this one is tested thoroughly. All features are important to test, as not only Lightning code has changed, but also the Mozilla Platform behind it.
If you find an issue, please file a bug at bugzilla.mozilla.org, product Calendar. Search for an existing bug first. If you come across any major bugs (not missing features), a comment is fine too.
Thank you for your support!