Grab a decent-size window with a decent-size page (e.g., <https://hg.mozilla.org/mozilla-central/rev/c9efa53d7aa9>). Drag it to and from your external display from your MBP. Watch how the contents of the window flicker between half-size (immediately after moving to hidpi) or double-size (after moving to lowdpi) and the correct scaling. Testing with an old 30" Cinema Display and a late-2012 Retina MBP, and 2013-04-21's Nightly.
I'm not clear how other apps do this, but they definitely don't flicker.
I believe this has been fixed elsewhere. I saw this in 2013 but it has been long fixed.