The problem
I just found a very strange issue with background-images in Firefox and Opera: the rule simply doesn’t get computed.
The rule doesn’t get shown in the inspector list, the images in background doesn’t get even loaded in network monitors. But everything is ok in the css file being loaded, tough… the rule, the selector, everything just fine.
The same page works ok in all other browsers, except these two.
Wonder if anyone had this experience earlier. Thanks.
Examples
I could manage to reproduce the error in jsfiddle.net as suggested by Vucko and monkey. I got two fiddles: one working and one crashing. The crashing one is more closer to what I have here in context. Hope it helps.
PS.: Just note that the crashing one actually works in browsers other than Opera and Firefox.
Just remove the last line in your CSS code.
#myprojectname > #page1 > .beginning:after > h2 > div.Tested in opera and mozilla.