I have an application that opens in a new tab in Firefox, but that is what I have for the default setting in Firefox. I probably should have quantified it in that way. I have never tried it in IE, but I would assume that if it was set to use tabbed browsing that it should work. If the default setting is to open a page in a new window, it is not going to override it though.
And if not, hey, it wouldn't be the first time I was wrong today....
I see a lot of programmers using Javascript to open pages in new tabs; again, it is still not gong to override the browser's default behavior.