For those who don't follow the Web-Tech blog --- you should. But anyway, support for SVG filter, clip-path and mask on non-SVG content landed on Gecko trunk a while ago and is in Firefox 3.1 beta 1. Also, I've proposed these extensions to the SVG WG for standardization.
Even more exciting is that Boris Zbarsky did an awesome job of implementing external document references --- I'm not sure if that made beta 1, but it will definitely be in beta 2. This means that all code that uses nsReferencedElement to track which element is referenced by a given URI/fragment-ID now automatically supports referring to external resource documents --- i.e. URIs of the form foobar.xml#abc. And Robert Longson has done a great job of migrating our last remaining SVG URI-ref users to use nsReferencedElement --- that is, markers and textPath --- so as of today, all the places where we support SVG referring to elements by ID support referring to elements in external documents as well as the current document. (It also means they're all "live for ID changes" and safe to use with incremental loading of SVG documents.)
The combination of these features is particularly cool because it means you can now apply SVG filter/clip-path/mask in regular HTML (non-XHTML) documents by placing the effect definitions in an external SVG XML file.
We're pretty much done for new features in Gecko 1.9.1 at this point. Looking forward post Gecko 1.9.1, we will be able to build on the external resource document loader to support SVG fonts (including via CSS @font-face) and SVG images (for CSS background-image etc, and HTML <img>). They should be a top priority for Gecko 1.9.2 or whatever it ends up being called.
At this point most of my "bling branch" has landed, except for two features: SVG paint servers (gradients and patterns) for non-SVG content, via CSS background-image, and the "use any element as a CSS background-image" feature. I'm not sure what to do with them. The former probably should land at some point, but it's not a high priority for me at the moment --- maybe I'll roll it into SVG background-image support, since they're closely related. For the latter, my current thinking is that some uses are adequately served with a CSS background-image referencing an SVG pattern containing a <foreignObject>, and other uses really demand an API that lets you specify a particular DOM node to render (e.g. to mirror a particular element in a particular IFRAME).
For that case, I think the way to go is to to create a new element --- some sort of viewPort element that acts like a replaced element and renders the content of some other element. It would have an attribute href that lets you declaratively specify a URI to the element to render, but it could also have a setSource(node) API so that you can give it a specific DOM node to mirror. You could even have an allowEvents attribute that lets events pass through the looking-glass... Right now MozAfterPaint and canvas.drawWindow are the best way to do effects like that, but they're not optimal. (Although there are uses for MozAfterPaint that the putative viewPort element would not satisfy, such as paint flashing/logging for debugging tools.)