Are you referring to the quick filter... the icon looks like a magnifying glass. That remains between searches in my experience.

Are you referring to the global search that is the space on the mail toolbar. I see no fields options.

I doubt you refer to the search under (Ctrl+Shift+F) but will include it just so we get the answer against the right one.

Actually I'm referring to thunderbird email program that has replaced Eudora on my windows 7 laptop. This is a stand alone program and not part of a browser.

I write click on a mailbox and choose search messages. Before I can search, I have to choose which part of the messages to search, body, subject, from, etc. I can add a field called everywhere but there doesn't seem a way of defining it.

I take it your technical skills are extremely limited. Other wise you would have answered my questions.

From the limited additional information you have supplied I am guessing you are using the quick search. It is the only one with a Sender Recipient Subject Body filter under the text input area.

As I said earlier, settings there remain between Thunderbird sessions. That is unless you use something like CCleaner that deletes the settings and calls it protecting your privacy.

See http://thunderbirdtweaks.blogspot.com.au/2012/06/thunderbird-and-ccleaner-or-my-settings.html