• About the non working video playback please file a bug.
• There should not be nss/nspr dependency on updates-testing if the system is updated (dnf update nss nspr)
• If some addons are not working they probably are not compatible with 55. We can't do much about it.

This should not depend on nspr update, which was undesired. See https://bodhi.fedoraproject.org/updates/firefox-55.0-5.fc26#comment-645713 for details.

@hreindl Did you settle your difficulties? If not, please file a bug for that.

LGTM

This update has been unpushed.

skarale: you also need to install thunderbird 45.1.0-2. gdata package depends on thunderbird package. So: sudo dnf install http://kojipkgs.fedoraproject.org//packages/thunderbird/45.1.0/2.fc23/x86_64/thunderbird-lightning-gdata-2.6.0.3-2.fc23.x86_64.rpm https://kojipkgs.fedoraproject.org//packages/thunderbird/45.1.0/2.fc23/x86_64/thunderbird-45.1.0-2.fc23.x86_64.rpm should work.