Rated 4 out of 5 stars

Whereas browsers gravitate towards a single search-and-URL field, Thunderbird decided at some point to have two search fields. I can't understand that weird decision, but fortunately Unified Search exists, with the Unified Search Widget that provides both quick search (filtering in the current folder) and global search in the words database.

Great work! I'm giving it four stars because the current beta version 0.7.4 doesn't completely work in TB 31.3: some user interface elements are broken. Specifically:
- The funnel icon ("filter") in the Unified Search Widget becomes invisible when filtering is off. It took me a long time to discover it was still there and I could still click it to turn filtering on. (If you don't know that and filtering happens to be off, the Unified Search Widget becomes almost useless - hence davidu's remark in his/her review below that "Even 0.7.4 is not working with TB 31.1").
- The filter options in the horizontal bar below the Unified Search Widget don't change color when they're on or off. So you can't see whether you're filtering on sender, receiver, subject, body, or any combination of them. (To see it you must convert the bar to a menu, but the bar is much more convenient).

Thanks IagoSRL, and I hope you can iron out these minor problems.

This review is for a previous version of the add-on (0.6.4).