As of last summer, my experience was that Firefox (and IE) would render text in a fallback/default font if waiting for another font to download. Has this changed?
(I suppose you may get web page layouts "jumping" if the downloaded font had very different spacing to the fallback font. EDIT: Ah, just learnt this is called FOUT. http://www.paulirish.com/2009/fighting-the-font-face-fout/ Still, I'd personally prefer FOUT to being unable to read the page.)