I got this referenced from another list.
There is an add on that allows folder navigation by typing the first letter and it is called 'quick key folder navigation.' The description says you have to type the first 3 letters, but if you only have one folder that begins with the letter key you press, then you will move right to the folder with just the first key press. I have installed this and it works great. This restores one of the favorite features for screen reader users that existed in the venerable and now largely defunct Outlook Express.
Using Thunderbird, it is easy to install this add on. Just click tools/add on manager/search for add on. Type in quick folder key navigation, press enter, then tab to the list of results, highlight the add on, right click and choose install. Then follow the prompts. There is no need to browse the Mozilla website for the add on.
They Ask Me If I'm Happy; I say Yes.
They ask: "How Happy are You?"
I Say: "I'm as happy as a stow away chimpanzee on a banana boat!"