Weird, unexpected(?) CSS behaviour

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Here's how to (try to) reproduce the issue: Open Firebug on any site and look for selector body:not([id])

• give it a new property and value (like visibility:hidden)
• note that nothing happens
• refresh page and you'll have it

From now on, you should see it now implemented on any refreshed or new tabs; moreover, even on different Firefox windows and versions (I have both 64- and 32-bit). Note that you'll only see said property/value if it's not specifically defined.

To revert, you need either to disable or delete the property, then refresh the page. Or restart Firefox altogether.

I don't see this is very useful to work this way. Could anyone confirm this? There's of course the possibility that Firefox itself or some other extension is the culprit. Or my rig being messed up somehow. As merely a graphic designer I'm not that much in to coding anyway. And sorry for the trouble if this is already been reported and I just missed it.

My Firefox version is 54.0.1, Firebug v. 2.0.19. Other extensions include BetterPrivacy, Custom Style Script, Disconnect, HTTPS Everywhere, Privacy Badger and NoScript. My OS is Win10 Pro. Other details on request.