When Firefox displaying a PDF file, the text "[Anlage: Note]" is displayed instead of an embedded annotation. Before version 112.0.2, a uniform icon (note) was displayed, unfortunately also not in the color specified, but always in yellow.
I was going to ask you for an example PDF, and that might still be helpful, but I created a simple one for testing, which is located at https://www.jeffersonscher.com/res/pdf-popup-annotation.pdf
When Firefox tries to load a certain image, it uses a path on the host website instead of in the program code, which of course is not valid, so then it displays alt text where it doesn't belong.
I filed a new bug report about the text here: https://bugzilla.mozilla.org/show_bug.cgi?id=1830645