FF 45 has only very limited WebExtension support. Some necessary API which is required by Tampermonkey is faulty or even not there.
That's OK because the WebExtensions API is in an alpha state at this Firefox version, but this causes TM to require Firefox 46 to work. Sorry.

https://blog.mozilla.org/addons/2015/12/21/webextensions-in-firefox-45-2/