Rated 5 out of 5 stars
As others have stated, this extension should be an integral part of Thunderbird. In my case, the Thunderbird built-in searches were slow and less than successful, and lacked the capability of more complicated searches.
I have been using it for a number of months and have not run into any issues.
For those people who indicated that "it doesn't work" or something similar, remember to toggle on View->Toolbars->Quick Filter Bar, otherwise you will not see the correct filter field in which to enter Expression Search (ES) functions and parameters.
Also, for those people who decry the lack of a global search or ability to search subfolders, all you need to do is create a Saved Search folder and run ES on that folder. You can modify the folder on-the-fly by right-clicking it and choosing "Properties."