In versions of Firefox prior to 4.0 there was an svg.enabled flag that you could set to false in about:config to disable Firefox’s SVG capability.
During the development of Firefox 4, UI changes took more and more advantage of SVG; for buttons for instance you can ship with fewer bitmaps – one scalable drawing can replace all the bitmaps for different screen resolutions and using SVG filters you can even derive the greyscale disabled state from the enabled button.
Eventually we discovered that the Firefox 4 UI had become so internally dependent on SVG that it would not start any more when you set svg.enabled to false so we removed the
flagfootgun. SVG is now a first class citizen just like html.
One consequence of this is that if you were using an SVG plugin such as the Adobe or Corel SVG viewers these will no longer function. To ease the pain, we have implemented more of the SVG specification in Firefox 4 than ever before and as you can see we’re up to a similar score to the Adobe plugin. There are still some things that the Adobe plugin does that Firefox does not, such as SVG fonts there are now things that Firefox does or does correctly that may in some way make up for that.