Here is the original bug report ^
I am resubmitting this to add the following:
The contact class contains a set friction and reset friction method. When you change the friction on a fixture, you then iterate through all contacts for that body and call reset Friction. You can do this at the time of friction change (best) or during presolve or really at any time. The contact class can be used to manipulate a lot of things, including overriding default mixtures for elasticity, density, friction and so on. You can even use it to ignore collisions on a frame by frame basis for oneway collisions and also use it to create tangent forces/impulses.