Question: I know target="\_blank" is supposed to make the thing open in a new window, and for some browsers, like FF3, it will actually make it open in a new tab. Is there a way to exercise more control over this behavior as a developer?

Answer: 
Currently the final decision on how to open a link is in the hands of the user and their browser.

Also, I wouldn't be too happy if a developer overrode my choice in the way that you are describing. I only ever want one browser window open at a time, and do not want child windows to appear. Other people feel much the opposite. We should try to respect that as developers.
Whether something targeted to "\_blank" opens to a new tab or window is a browser/user specific option settable in most new tab supported browsers. There is no way to target a tab yet.