Mozilla’s Robert O’Callahan has posted a few hints about new features expected to land in the next version (1.9.2) of the Gecko layout engine.
Allowing use of SVG in <img> and background-image is high on the list, apparently, as are background-size, multiple backgrounds, and possibly text-overflow. These features have already been implemented between Opera and Safari.
I can’t find any information yet as to when Gecko 1.9.2 will land; I think it’s doubtful it will make it into Firefox 3.1, so are we looking at 4.0 or an as yet unannounced 3.2?