It's not displaying alt text in firefox because firefox is compliant with standards. For it appearing sometimes there is very likely a reason but I am unable to have it done.
Quote:
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The alt attribute specifies alternate text that is rendered when the image cannot be displayed (see below for information on how to specify alternate text ). User agents must render alternate text when they cannot support images, they cannot support a certain image type or when they are configured not to display images.
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See here:
http://www.w3.org/TR/html4/struct/objects.html
You should rather opt for javascript or css tooltips.
http://trentrichardson.com/examples/csstooltips/