Give a 👍 if you'd like to see DomFlags ported to FireFox.
Edit: Maybe it's worth linking this repo on http://domflags.com/ for more discoverability.
@Florian-R Added to the bottom. Thanks.
I actually started some time ago to hack something like this in the FirefoxDevTools, but never got to finish it.
My approach was a little different though, you could only flag nodes from the tool itself. The tool would find a way to identify the element uniquely by building a complex-enough CSS selector and would remember it, so that it would persist across page navigation.
I think your approach (with the domflag attribute) makes more sense.
I'd be happy to make a similar extension for FirefoxDevTools.