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As others have said already, mnenhy doesn't work with recent TB versions anymore.
There is a bug report about it on mozdev, and somebody posted a fixed version. I've tried it on Win10 and Linux (both 64-bit) with the latest TB version (45.7.1), and it works fine for me.
I've tried to post a link to it here, but that vanished. It can be found with a Google search for "stanio mnenhy". Or look up bug number 26220 on mozdev's bugzilla. Make sure you install the mnenhy-0.8.6.1.2-stanio.xpi version from 2016-07-10.
Maybe the link stays up now: https://bitbucket.org/stanio/mnenhy/downloads