Well, as far as having a completely and truly portable Chromium-based browser,
we do have one, Cent Browser ! My quest was just about trying to avoid a dependence on only one option.
Current (latest) Iridium, SuperBird, and Coc Coc, are also fully portable,
but that is likely only because they have not yet upgraded to Chromium 58 (where the "screw up" seems to have happened).
I expect these browsers to go down the drain in terms of total portability, the way Chromium, Iron, etc did.
I tested the FireFox 57, nightly build, that fully introduces the new FireFox (multi-tasking, web-extensions, new GUI),
and it looks and feels fantastic ! It will be the "official" version by the end of November 2017.
At the end of the year, I will dump any Chromium other than Cent Browser.
Chromium is like an irresistibly sexy girl-friend, but really "bad" - selfish, manipulative, controlling, always gets things her way.
By transitioning from old FireFox to new FireFox, I will loose a few priceless add-ons, I will shed some tears over that,
but I will still have my good old Pale Moon (version 24, 64-bit, portable) to comfort me in these sad moments of separation.
Also, maybe Pxc FireFox, WaterFox, and CyberFox will come up with ways to preserve the old add-ons
together with the new web-extensions, although at the moment that seems pretty impossible to me.
Otherwise, these "performance-enhanced" FireFox variations will become completely irrelevant with the new official FireFox.
JohnTHaller, You may want to update Your pages about the "limited portability" status of Chromium browsers,
the way PortableFreeware did with the Vivaldi browser "https://www.portablefreeware.com/index.php?id=2869
(I guess, an update on the rest of the Chromium family is pending) .
Just a little disclaimer - to avoid the "wrath and fury" of those charming trolls in the world of web comments.
Keep up the great work, this whole post was an effort to make the (Chromium) world more "Haller-like".