Something that has always bugged me about Firefox is that if it encounters a broken image it doesn’t display an image place-holder. Instead it displays the alt attribute as in-line text. This can cause problems if your primary development browser is Firefox, as you may not notice broken images on a page.
For a while there has been an option in the config to display image place-holders while a page loads but not for broken images as on IE (yes I’m praising an IE feature!).
Image placeholders on load is set to ON by default, your can change this if you want by going to about:config and searching for ‘image’. The option your looking for is:
browser.display.show_image_placeholders
Just double click to change the value.
As I said before, this doesn’t affect broken images after the page has loaded, and after several searches it looks like the option just isn’t available to Fx users as a general setting. There is however a solution…
Firefox allows users to specify custom CSS to be applied to websites on a global basis, e.g if you want your default link colour on unvisited links to be black instead of the default blue.
You do this by making changes to your global content css file. You can find it at the following location (OS Specific – I’m on windows 7) ‘%appdata%\Mozilla\Firefox\Profiles\\chrome’ (if your on a domain you will need to edit it in your roaming folder), look for a file called ‘UserContent-example.css’ and rename it to ‘UserContent.css’, this will then be loaded by Firefox when it fires up.
Add the following CSS to the file and restart Firefox..
/* Enable image placeholders */
@-moz-document url-prefix(http), url-prefix(file) {
img:-moz-broken{
-moz-force-broken-image-icon:1;
width:24px;
height:24px;
}
}
Thats it… when you next come accross a broken/missing image, you will get a box the size you have defined in its place with the alt inside it.
Update:
I was asked if I could add a broken image in this post as an example to see the css tweak in action, so here it is -> 
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