One of the most missed feature is the lack of using a linear colourspace when it is necessary, like scaling (especially scaling down) images [1] or calculating physical models (like the Phong model [2]) that implicitly imply a linear colourspace. Iceweasel 35 still doesn't use a linear colourspace for scaling, there is a simple example image showing that [3] (try to zoom out). Last time I checked their SVG engine's Phong model implementation (yes, SVG supports bump maps with custom lighting settings) was broken too. It was a long time ago, I submitted a bug report for it but somehow it was marked as a duplicate for a "fixed" bug so I do not think that it got fixed since then.
[1] http://www.4p8.com/eric.brasseur/gamma.html
[2] http://en.m.wikipedia.org/wiki/Phong_reflection_model
[3] http://www.4p8.com/eric.brasseur/gamma-1.0-or-2.2.png
edit: formatting
edit: Found the reference renderings of the feDiffuseLighting filter [4]. In my Iceweasel 35 the SVG images are much darker than the prerendered png.
[4] http://www.w3.org/Graphics/SVG/Test/20110816/harness/htmlObj...
It has python implementations of a ton of colorspaces and transforms. Also some cool support for spectral data.