Fortes You're correct that it will throw off spacing if the actual browser font size gets modified. However, the latest versions of IE and FF will not just increase font size, but apply zooms to images and other elements, so your page may work just fine (I haven't investigated).
For FF2 and other browsers, you can use jQEm to set up a handler that will re-run the script whenever the text size changes.
For FF2 and other browsers, you can use jQEm to set up a handler that will re-run the script whenever the text size changes.