Does Firefox display or have a setting for displaying Alt text? Can't find anything on it in the help files...
If you're looking for cross-browser behaviour when you're developing sites, use the title attribute instead of, or in addition to the alt attribute.
Added: In terms of W3C standards, Jetboy is right. The alt-tag is only supposed to be seen when the image is not seen. Title would be the proper thing to use as a web developer.