Rated 5 out of 5 stars

It looks like it would be extremely helpful but I don't see the buttons when I install it. I thought I might have to add them manually but there are no new ones after I add the extension. Maybe I'm doing something wrong? A previous reviewer said it was working for him.

Update - Thanks to the developer! It wasn't obvious from the description or screenshot where to find the buttons, but once the developer pointed this out, I love it. It really helps to keep me focused on what I have to do now and not let myself get distracted by older things in my inbox.

Those buttons are integrated into the Quick Filter Bar. You have to activate this bar (View / Toolbars / Quick Filter Bar or top "Quick Filter" button for latest thunderbird versions).