Neither the Thunderbird add-on you can get from Mozilla nor the Mozilla apps enhancement add-on for NVDA has anything to do with first letter navigation in the address book.  You could use first letter navigation in the address book previously because it was a standard list.  The list itself no longer supports first letter navigation.  You must search for an address using the search field you are placed in when you open the address book, then tab to the list to see the results.  It works like a filter and shows relevant results.

I never use the address book in Thunderbird except to add or remove addresses.  Autocomplete is much more efficient to use when finding an address to use in composing a message and that is true of the current version and of previous versions.

Gene

I have not seen anything that suggests this is the case.  I've never even seen a request for such on the developer's GitHub issues page for Mozilla Apps Enhancements.

First letter navigation in the mail folder tree is provided by a Thunderbird add-on, Quick Folder Key Navigation.
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Brian - Windows 10, 64-Bit, Version 21H2, Build 19044  

The real art of conversation is not only to say the right thing in the right place but to leave unsaid the wrong thing at the tempting moment.

        ~ Dorothy Nevill

So when the thunderbird enhancement addon is updated will we then be able to use first letter navigation in the address bbooks?