[emacs-w3m:11376] w3m-view-url-with-external-browser, browse-url-new-window-flag

I wish M, w3m-view-url-with-external-browser, would start in a new tab, like
$ iceweasel -new-tab URL

But we notice
browse-url-default-browser
...When called interactively, if variable `browse-url-new-window-flag' is
non-nil, load the document in a new window

OK, but we want a new tab and not a new window (-new-window). So there
ought to be a choice.

And I set browse-url-new-window-flag to t and M doesn't give a new
window anyway, at least with my http://jidanni.org/comp/configuration/
and emacs-w3m-version "1.4.400" here with Debian emacs-snapshot.

Anyway, ^UM should also act different than M.
It should act like
browse-url-default-browser:
A non-nil interactive prefix argument reverses
the effect of `browse-url-new-window-flag'.