Please do "Update" in GSlapt and try again.
If that fails, post the output of the command:
grep ^SOURCE\= /etc/slapt-get/slapt-getrc
Firefox 27.0.1 in the testing repo requires libpng >= 1.4.8, which was already installed here and is available as libpng-1.4.8 in the repos.
NOTHING in the 7.0 repos requires libpng 1.4.11, and the only "1.4.11" that's a package requirement is "gnupg >= 1.4.11", which is needed by playonlinux and gpgme. Gnupg 1.4.11 (and for that matter gnupg 1.4.13) is available in the repo.
This post powered by Firefox 27.0.1 on 32-bit Vector Linux 7.0 Light.