Was that with Global Search (Ctrl+K)? Global Search omits Trash folders by default, although it can be enabled in folder Properties (context menu). Try using Classic Search (Ctrl+Shift+F) on Trash.

sfhowes - thanks. Control-Shift-F brought up the great search and I found what I was looking for. How do I make that search the default one?

There is no search function that can be set as default, but you can add a button to the Mail Toolbar for one-click access to Classic Search:

https://addons.thunderbird.net/en-US/thunderbird/addon/search-button/

that's good, Thanks.. I just have to pick the folder I want to search in and it's ALWAYS the trash folder.

you da man..or da woman!

To search a single folder, it may be faster to use the Quick Filter Bar: Ctrl+Shift+K, enter some text, then click the filter buttons Sender, Recipients, Subject Body.

uh oh.. I tried experimenting. I have a friend who's name is Tom. I went to the 'search messages' box and clicked on 'sent' folder. I've sent him tons of emails. Clicked on search subfolders and then 'from' begins with' and 'Tom. Nothing came up whether or not I ticked the box that says 'search subfolders' or 'run search on server'. then I went to the search bar (control K) put his name in there and the emails came up.

In the Sent folder, all the From entries are you. Select To in the filter drop-down, then 'begins with' or 'contains' Tom.

AHA! got it.. so which would you say is the better search tool: the add on or the regular search box?

I think the add-on is just a button shortcut to the Classic Search. Classic Search and Quick Filter Bar are both useful for single folders; the former has more options, while the latter is simpler.