The conversion for "regular" <tabs> elements is straightforward, most of
the work here is to support the tab strip (the <tabs> element with the id
"tabbrowser-tabs"). The markup needed for the tab strip moves directly
into browser.xhtml and the construction/teardown logic is now explicitly
driven from gBrowser. There are many more little tweaks too numerous to
enumerate.
Differential Revision: https://phabricator.services.mozilla.com/D32855
c124590602