Annoying Mozilla bug that the people who control the tree won't fix:

Bugzilla Bug 25537
alt text is not displayed as a tooltip

The principle behind marking it VERIFIED WONTFIX is correct, but cruel reality intrudes.

Sigh.

No matter. Two people included workarounds using The Proxomitron! Number 2 seems to work better for me.

Workaround filter 1:
Name = "Tooltip fix - disregard tag with correct attribute"
Bounds="<img\s*>"
Match = "\1 title=\w\2"
Replace = "\1 title=\w\2"

Name = "Tooltip fix - add correct title attribute"
Bounds="<img\s*>"
Match = "\1 alt=(\w)\2\3"
Replace = "\1 alt=\2 title=\2\3"

Workaround filter 2:
Name = "Tooltip fix (change lone alts to titles)"
Bounds = "<img\s*>"
Match = "(^*title=*)"
"&"
"\1 alt="\2"\3"
Replace = "\1 alt="\2" title="\2"\3"

Or you can install an XPI which might or might not work: http://white.sakura.ne.jp/~piro/xul/_popupalt.html.en