If you use ubuntu-repo firefox, maybe you’ll get this error I found, amongst many curious posts, that such behaviour is caused by Unity Desktop Integration (have a look in Firefox extensions)… So, it’s easy, just disable it and restart Firefox then try to add a plugin, from Mycroft site for instance And it works!
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This post first appeared on Freelabs | Liberi Cantieri Di Sbarrax, please read the originial post: here