Firefox (and Thunderbird) cannot connect to any google service with nss-3.28.
This has been discussed in gentoo https://bugs.gentoo.org/show_bug.cgi?id=603622
and patches in comment 5 fix the problem. I confirmed by rebuilding the latest
firefox rpm with the patch.
Wikipedia has the same issue with this update.
New builds are in koji - http://koji.fedoraproject.org/koji/packageinfo?packageID=37
Working with firefox-50.1.0-3.fc26.armv7hl, no longer seeing 'NS_ERROR_NET_INADEQUATE_SECURITY'
Should be fixed now.