Turns out there is a much easier way to edit Thunderbird message tags. I was not even aware there was an option (albeit a little buried) until FreeJeff left this informative comment:

You can configure tags to your heart’s content by simply going to Options > Display > Tags. Add, delete, edit title and/or color, whatever. About:config is completely unnecessary.

Oh, so there is a ‘Tags’ option in the Display settings. Whoops, guess I should have looked thru the options menu first.

Thanks for the info FreeJeff!