[7kay]

I know of expression-search(https://github.com/wangvisual/expression-search) but it is not supported in recent versions of Thunderbird. I'd be interested as well as I've been longing for a proper keyboard UI. For quickly switching folders there is Nostalgy(https://github.com/nostalgy/nostalgy).

[kioleanu]

Yes, expression search. It works with TB > 68 with a little bit of tweaking. The details are in the top issues