Tampermonkey was updated today. Sorry for any inconvenience.
Is the button menu text sized in a normal way now?

Edit: The menu should be sized correctly once issue 1215025 [1] is fixed. At the moment a minimum size is set to make the menu usable for now, but it can't be too large to not overwrite the correct size at later FF releases.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1215025