Being unable to wait (well, it's almost tomorrow)...
Vanilla Fx 2.0.0.8 w/only B2T 5.0 - works more or less correctly: no icons in the Customize process, but they do show up on a restart. Plus, I can drag the icons on the status bar. So, one of the half-dozen other extensions either is causing my couple of problems with no-drag & no-show of icons (either not compatible or the order of install--same diff... more testing to come). Still doesn't work with CuteMenus - Crystal SVG as far as icons showing up in the context menu (bummer).
(OK, now it is tomorrow)
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Well, installing Stylish removes the ability to drag the icons on the status bar.
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Completed testing with the other extensions--no joy in isolating a particular one which causes the "lost" icons/buttons in the toolbar (there's something there and you can click on it, but it's a very narrow "canvas"). I did restore the CuteMenus icons in the context menu using the CSS code suggested by Chuck (put it in a Stylish script actually

). Makes me wonder if the two "disappearances" have a related cause (?). My other Firefox's are still using v4 of B2T with no problem, so I'll just leave them alone for now.