Also see bug 1274721 for Thunderbird and bug 1274722 for SeaMonkey for reference In bug 1271761 a new value (%SYSTEM_CAPABILITIES%) will be added to the update url so it is possible to prevent updating of Windows systems that don't have SSE2 support. A fix for this should not land until after the dependent bugs are fixed as well as the Instantbird update has support for the new value. This will also need to be uplifted to Mozilla 48 and ESR if applicable for Instantbird.

On the behalf of Florian: Closing bugs related to the Instantbird UI as WONTFIX, as the development of the standalone chat client Instantbird has stopped. Instantbird users are encouraged to migrate to Thunderbird. The user interface of instant messaging in Thunderbird will feel familiar, as the Thunderbird IM support started as a fork of Instantbird.