The security of Firefox/Iceweasel is important for the security of Tails.
I therefore suggest to have a look at this old unresolved Firefox issue
and vote for it. Years ago people working for RedHat spent a lot of time
to create a patch which does not yet seem to have been applied.
Resolving the issue would make Firefox more secure (even when SELinux is
not used):
SELinux is preventing JIT from changing memory segment access
https://bugzilla.mozilla.org/show_bug.cgi?id=506693
Cheers,
Andreas