I spoke too soon about WB8 final fixing everything ...
I visited a site today I hadn't visited in a long time and attempted to download something -- when I noticed that the radio buttons no longer display in the dialogue box:

So, I unloaded WB and saw that they did then display:

I tried several different themes -- and got the same results each time.
Thinking this might be a conflict with my recent upgrade of Start8, I uninstalled Start8, rebooted, tried again -- and got the same results.
So I tried all the same (with and without Start8) with WB 7.9 -- and got the same results.
These are dialogue boxes from FF v22.0. I tried to replicate this with Chrome and with IE 10 -- but they don't put up dialogue boxes with radio buttons, only FF does that.
UPDATE: Remembered that I had setup FF to do automatic updates, so I uninstalled FF v22 and reinstalled FF v21 -- and the problem has vanished! So, apparently, this is a problem with FF v22.