When an image fails to load on a page, the behaviour of Firefox seems to be to show its alt text if that attribute exists, otherwise it shows the broken image icon. This isn't ideal as the alt text attribute can exist but be empty, leaving a blank space which the user may not even realise is supposed to be an image. Even when the alt text attribute isn't empty, it might not be immediately apparent that it's actually alt text for an image and not part of the webapge body's text etc.
Instead of showing just the alt text, I suggest showing both the broken image icon and the alt text, like Chrome's behaviour. Preferably make this a configurable setting so the user can choose whether to implement this feature or not.