It was something like no alt text being rendered when images are not displayed or found, I don't fully remember and could not find it in 5 minutes Bugzilla search hell. I did stumble on the still-open bug some days ago though, I think it was 8 years old now.
Is this what you were talking about? https://bugzilla.mozilla.org/show_bug.cgi?id=45375 - TITLE text is truncated.
Resolved in 3.0, but was outstanding for many years. The bug comments are worth reading (some of them are from the XKCD and Dinosaur Comics guys, whose sites this bug impacted fairly heavily). It doesn't make their dev community look like a place I'd want to be.
What is correctly shown? It shouldn’t be shown as a tooltip as that leads people to write alt text that’s specifically useful as a tooltip and useless to the real consumers of alt text: screen reader users and search engines.