From MozillaZine Knowledge Base

Most likely, you are experiencing the situation where ALT attributes don't become tooltips when you hover over an image. This is because this way of handling ALT attribute is incorrect. ALT text should only be shown as an alternative to the image, not as well as the image. To have the text display in the tooltip in all graphical browsers, the TITLE attribute should be used. Contact the site administrator to have it fixed.

From w3.org:

Several non-textual elements [...] let authors specify alternate text to serve as content when the element cannot be rendered normally. Specifying alternate text assists users without graphic display terminals, users whose browsers don't support forms, visually impaired users, those who use speech synthesizers, those who have configured their graphical user agents not to display images, etc.

If you want Firefox to show ALT attributes in tooltips, install the Popup ALT Attributes extension.
You may alternatively install a user script titled ALT Tooltips For Firefox in conjunction with the Greasemonkey extension.