Hmm. I don't have a good solution for that. It would be trivial using an extension like Stylus though, just don't allow it to run in PB mode.

I suppose you could do all your @-moz-document matching with regexp and then using a bookmarklet to add some "mark" to the document url. Then you wouldn't need to use PB-mode at all.

So, a bookmarklet like this, when clicked, adds/removes a#pbm suffix to the current tab and loads that address to a new tab:

javascript:((loc)=>window.open(loc.endsWith("#pbm")?loc.slice(0,-4):loc+"#pbm"))(document.location.href)

Then in you userContent.css you would write your document matching like this (example is for en.wikipedia.org):

@-moz-document regexp(".*en\.wikipedia\.org.*(?<!#pbm)$"){
  body{ color: red !important; }
}

That should work but honestly if you need this then I would rather just use extension like Stylus or open those links in separate profiles even.