There currently is a Tampermonkey update in the review queue, but it might take some time until the review gets finished. (https://github.com/Tampermonkey/tampermonkey/issues/352#issuecomment-285466789)

Nightly and developer version users can use this unofficial release in the meantime: https://tampermonkey.net/xpi/46544b6d654f61/firefox-5410beta.xpi

Edit: 5410beta is working fine here with Nightly 55.0a1 (2017-03-23) (64-bit) , Developer Edition 54.0a2 (2017-03-23) (64-bit).

Edit: ... and 55.0a1 (2017-03-29) (32-Bit) from here: https://download-installer.cdn.mozilla.net/pub/firefox/nightly/latest-mozilla-central/firefox-55.0a1.en-US.win32.installer.exe

I just installed this and Tampermonkey and everything is working fine. Can you please create a new temporary Firefox profile and check whether it's working there?