@jack, roger, stephen, aaron, tim, james, and any others I may have missed, thanks for all your efforts.
@list, sorry for the noise.
Once again the problem existed between the chair and the keyboard (pebcak).
I moved .mozilla out of the way and restarted firefox which recreated a clean .mozilla directory. iframes worked. So I went file by file, dir by dir looking for a difference that could explain what was happening.
Got to /chrome/ and found the file userContent.css where the resident genius had added the line iframe {display: none;}.
Sometime ago in the wayback I was experimenting with using iframes as a client/server communication channel and since there was no need to see/style them I hid them from myself (which also let me explore customizing firefox). By the time I had finished with the experiments I had completely forgotten about userContent.css.
It's taken months to figure out what I'd done. Sure made online-banking/bill-paying/etc. an endeavor every time I tried. If it weren't for firebug I'd have been back to writing checks. aaaargh!
Thankful, sorry, and embarrassed, Mike Wright