The porting effort for software to run on x32 is a rounding error compared to what would typically be required to universally use offsets instead of pointers in dynamically allocated objects.
Something like a JVM can do this reasonably easily because the JVM bytecode itself is pointer size independent. Making a comparable change to Firefox or Chrome without compiler support would be essentially impossible. It would amount to translating C and C++ into an entirely new dialect. The chances of the upstreams for something like Webkit merging that are basically non-existent.