Image tooltips do not work
From MozillaZine Knowledge Base
(Redirected from Firefox : Issues : Tooltips Don't Work)
Most likely, you are experiencing the situation where
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