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Please, need be compatible with version 57

Questa recensione riguarda una versione precedente (1.25) del componente aggiuntivo. 

FF56 will be the last version this addon compatible with, because mozilla is killing all extensions that require XUL.
https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/comment-page-1/
For now MP+ still runs fine in FF57 (you need adjust max version in install.rdf and disable xpinstall.signatures.required in about:config), but it will most probably change in the future and nothing I can do about it.

[EDIT]
It's no longer compatible with FF57 and never will, give all the thanks to mozilla for that.