macOS set image on about:blank page in firefox

So I'm trying to set an image in the about:blank page of my firefox browser. I found a post online on how to do this:

http://www.theyagar.com/2007/06/16/how-to-set-a-backgroound-image-in-firefox/

It sounded simple, I found the userContent.css file located in the following path:
Library/Application Support/Firefox/Profiles/*.default/chrome

I then edited the file adding the following command:
"-moz-document url(about:blank) {
html {
background: url("http://rakaz.nl/artwork/wallpaper/black/black-1600x1200.jpg") no-repeat center center fixed black !important;
}
}"

and things worked fine ......

However after quitting and reopening Firefox the about:blank page was, well BLANK. I've tried closing and opening afew times and sometimes I'm lucky enough to see an image and sometimes not. DOES ANYONE KNOW WHATS HAPPENING AND HOW I MIGHT FIX IT? Thanks

Cheers,
ScKaSx