[Ubuntu Firefox] "Your browser does not support OpenSearch search plugins"

If you use ubuntu-repo firefox, maybe you’ll get this error

Your browser does not support OpenSearch search plugins

I found, amongst many curious posts, that such behaviour is caused by Unity Desktop Integration (have a look in Firefox extensions)…

Unity Desktop Integration

So, it’s easy, just disable it and restart Firefox

UDI-disabled

then try to add a plugin, from Mycroft site for instance

addOk

And it works!