There have been a number of threads recently regarding problems with Quick Folder Key Navigation extension for Thunderbird. I just ran into the problem on a brand new laptop, when this extension doesn't work at all without producing any error messages, and I spent some time investigating the cause, so I just wanted to let people know about what I found just in case anyone else runs into this problem again.
It turns out that McAfee antivirus that was preinstalled on my laptop believes that this extension is a virus. Adding insult to injury this antivirus appears to be completely inaccessible with NVDA, so even though it shows an alert window, there is no way of knowing about that. As a result, the extension appears to be silently not working.
In order to fix this, you would need to uninstall McAfee antivirus (sighted help required) and reinstall the extension. This will also switch your computer to Windows Defender antivirus, which doesn't seem to have a false positive on this extension.
Further diagnosis of any problems with this extension can be performed by checking any errors in Thunderbird error console (Menu>Tools>Developer Tools>Error Console). In my case I had the following error message on startup:
Error while loading'jar:file:///C:/Users/tony/AppData/Roaming/Thunderbird/Profiles/6tp5u0gh.default-release/extensions/keynav@...!/manifest.json' (NS_ERROR_FILE_CORRUPTED) Extension.jsm:570
This error is happening apparently because McAfee antivirus replaces extension file keynav@... with an empty file, and Thunderbird is not smart enough to at least alert user of such error.