You will need to install a user script manager extension to install this script.
This script will force Firefox to show broken image icon when image failed loading, wether the image has alt attribute. You can press Alt+R to reload the broken images.
Image without alt. you should see the icon by default. <img src="https://i.am.broken"> Image with blank alt. By the default, you will see nothing. <img alt="" src="https://i.am.broken"> Image with alt attribute. You should see the text in alt attribute. <img alt="There used to be a image, but now it is broken" src="https://i.am.broken">
After installing the script:
If you just want to reload broken images, you can use the following code as the bookmarklet:
3.0.0 (Feb 17, 2021)
2.2.1 (Apr 4, 2016)