The problem is, that the version in openSUSE:Factory:Update/MozillaThunderbird
still contains the size optimization:
%if %{with mozilla_tb_optimize_for_size}
export CFLAGS="$CFLAGS -Os"
%endif
This supersedes the O2 optimization. Remove it and it works.
At least it works for me. Tested in home:hendwolt:branches:mozilla:Factory
on openSUSE TW x86 (in a virtual machine and on bare metal) and openSUSE TW
x86-64 (in a virtual machine only).