That feature works. You need to provide more information.

At the top of the page there used to be a place where you could do a search for things in your emails by typing in key words.

That search location is no longer on the email site.

It is still there. you can also use the Edit>find menu and there is also the quick filter button above message list. Maybe if you post a screenshot of your thunderbird pane, some ideas will surface. Thank you.