Sure: pay money directly to one of a thousand developers to implement missing functionality or fix pressing bugs in Firefox and make a pull request on it.
For example: I see that Firefox still hasn't yet made all SVG elements attributes into presentation attributes per the SVG 2 spec. So right now in Firefox you can't do 'rect.style.width = "12px"' but in Chrome you can.
Not sure you could easily get full coverage (e.g., I'd bet css "d" atty would be tricky). But there's probably a fairly workable introductory subset for which one could create a fairly clean patch set.
Or pick a different self-contained, non-controversial feature/bug and fund a dev to attack it.