Thunderbird add-ons (AKA Extensions) are installed from within Thunderbird itself, using the rough equivalent of NVDA Add-Ons Manager.
1. Hit ALT + T (Tools Menu)
2. Followed by A (Add-Ons & Themes)
3. In the Find more add-ons search box, enter the search term(s). In this case give the exact add-on name: Quick Folder Key Navigation
4. Hit Enter to perform the search. It's the only returned result.
5. On the add-on result screen, there will be a "link button," Add to Thunderbird. Activate it.
6. The Thunderbird add-on install process should run.
Brian - Windows 10, 64-Bit, Version 21H2, Build 19044
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