Sorry, you can't. SquirrelMail is made specifically with frames. It works well, however, to use the Mozilla Firefox browser's "tabbed browsing" to open up seveal mails or other pages in separate tabs of the same browser window.
If you're not using Firefox now, give it a try. You just might like it. (Sidenote: I used to be very pro-InternetExplorer but after trying Firefox for three days I have completely turned about.)
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