About a month ago I've switched from Firefox+Thunderbird to Opera+mutt. I got fed up with all too frequent security updates for Firefox. They get even more troublesome when you have to compile it yourself, like on FreeBSD. As for ditching thunderbird - I got fed up with it trying to move my existing emails from one version of TB + enigmail to totally different versions. I've spent 3 hours making it work!
Overall, I'm pretty satisfied with the switch. Opera has some stability problems - it crashes now and then. The good thing is that, unlike FF, it remembers all pages that were open before the crash. The restart takes just a few seconds, and Opera resumes seamlessly where it left off, so it's not even annoying.
Opera also has an integrated RSS, NNTP and mail readers. As I'm reading less and less news every day, due to time constraints, so I didn't even look at it. RSS reader is working, and I'm using it.
I'm not using the mail client because it currently does not support PGP. I didn't investigate whether it supports S/MIME. Another reason for not using it is the experience with thunderbird and moving archived mails around and between upgrades of TB. The option "import/export mail", assuming that it exists, is not present in the most obvious places. I've learned my lesson regarding archived mails, and I'm not going to repeat it.
With mutt, moving mails is straightforward - just move the mail folders directory. No stale setup in obscure places that needs to be cleaned up after an upgrade, as is the case with thunderbird.