Someone just showed me that Firefox changes alt text size (when viewing with alt instead of images). In IE all alt text on a page is the same size, a standard size determined by the browser/OS. But in Firefox the alt text is displayed in whatever font size is current for that bit of the page. Firefox also expands image "boxes" to fit all the alt text, whereas IE will keep the boxes at the specified image width and height, cutting off any text that is too long to fit.
The result? Firefox can cause unexpected horizontal scrolling when pages are viewed with alt instead of images (but at least all of the text is there!).