Can any Firefox users (Firefox 91 or later) on macOS (macOS 10.15 or later) confirm whether broken images will display alt-text? This article seems to indicate that alt-text was not displayed in Firefox 81 on macOS 10.15: https://adrianroselli.com/2020/10/alternative-text-for-css-generated-content.html

To test: navigate to "about:config" and set "permissions.default.image" to "2". Then load a page containing an image with alt-text, and see if the image is replaced by *visible* alt-text.
Remember to set "permissions.default.image" back to "1" when you're done.

I'd also love to see a screenshot if you're up for it! I've attached a screenshot of https://seirdy.one/posts/2022/06/25/two-types-of-privacy/ to show what it should look like.