The issue we had to solve is that GTK+ 2 and GTK+ 3 cannot be loaded in the same address space. Moving Firefox from GTK+ 2 to GTK+ 3 isn’t a problem, as only GTK+ 3 gets loaded in its address space, and everything is fine. The problem comes when you load a plugin that links to GTK+ 2, e.g. Flash. Then, GTK+ 2 and GTK+ 3 get both loaded, GTK+ detects that, and aborts to avoid bigger problems.