there most definitely is a way, its just a matter of figuring it out
with mozilla's everything-is-css-and-xml approach, you should be able to do a iframe { display: none; } somewhere, i'm just not sure where, or the exact syntax of it (xul css has some idiosynchrasies from run of the mill css)
in fact i've started wondering how i can tell mozilla to not display unloaded images (i.e. not display the empty "box" with the image icon in it, it's annoying, especially a page full of them while the page is loading)
try reading up on xul and userChrome.css on google, its interesting stuff. for example here is my rather simple userChrome.css:
#status-bar { font-size: larger !important; }
#throbber-box { display: none !important; }
.toolbargrippy { display: none !important; }
the first line makes the status bar text actually readable, the second line makes the mozilla "loading" animating icon thingie go away, and the third line gets rid of the grippies on the left of toolbars.
i just dont know if css for webpage contents goes here too, or if that's somewhere else.