It does what you need. Really don't understand why this isn't default behaviour.

This must be a very very old extension because it does not seem to work in any real way. It added a bunch of "null" alt attributes to just about every node in the HTML DOC. I was hoping for an add-on that I could customize the popup alt attribute style, make it have more lines (fixed width), and maybe make the text bigger so I can see it easier, perhaps have it centered on the mouse cursor instead of connecting the cursor to the yellow box at the top left corner, that way if I am using a magnifying tool I can see the entire popup box easier.

Oh well, the search goes on.

Excellent for quickly testing image accessibility when not using a screen reader.

So useful that I think Firefox should display alt text as a tool tip by default (unless a tool tip attached by other means such as the title attribute has precedence; or in addition to a title attribute of a containing element)

This extension does not work for the following pages:
en.wikipedia.org/wiki/Template:Span_title
en.wikipedia.org/wiki/Template:Abbr

Awesome in 2 ways:

1) Nostalgia. Reminds me of my days browsing with Internet Explorer when I was a kid. :)

2) I'm now learning to be a web developer and so I know the importance of adding alt tags to images. This feature gives me a higher awareness of how sites are coded while I'm browsing, and lets me easily see how the alt tag is being used around the net.

I'm glad this feature doesn't come built in to Firefox or other modern browsers because I know back when this feature was in IE and IE was popular, the alt tag was horribly misused for its popup qualities and its actual purpose was neglected. It's much better to have this as an add-on. I love it already.

I have to check alt tags and this makes it so easy!

Is there a way to copy the alt text?

Does what it says.
I keep realizing that without Piro's extensions my life in Firefox would be hard to imagine.