I finally got thunderbird working again by following the instructions in the same link,
http://www.avast.com/faq.php?article=AVKB91#idt_0301, but it took me two tries.
the only difference was the second time through i did a better job of deleting the avast!mail scanner entry under the servers tab, the step shown in figures V. C and VI. C.
first time i highlighted the main entry and hit delete, and even though nothing visibly happened i decided that was good enough. nope.
when Tbird still wouldn't work with avast running, i decided to run through the process again.
but this time, when i got to the servers tab i highlighted the avast entry and then clicked on its subfiles. when i hit delete on each individual file, i got a dialog box asking to verify its removal, and after the second deletion the avast! mail scanner entry disappeared from the list.
after i finished the rest of the process, thunderbird started working normally.