Work on Iceweasel 3.6 is approaching an end, but there are still issues with it and with 3.5 that I want to tackle before officially pushing to experimental.
In the meanwhile, you can already try a preview package:
The amd64 binary has the JIT compiler enabled by default (contrary to upstream default, and I guess most if not all other distros), so if you have issues with it, you’ll need to disable
about:config. It passes the testsuite, so I expect it to be fairly stable.