After upgrading to profile 17.1 plugins symlink is broken /usr/lib64/firefox/browser/plugins -> /usr/lib/nsbrowser/plugins/ should be pointing to /usr/lib64/nsbrowser/plugins/
Did you run all steps shown in profile 17.1 migration guide? You probably need to re-emerge all packages still using the old location. Please do that and report back if this will fix your issue.
firefox ebuild doesn't call share_plugins_dir() for a very long time, it fixes plugins path in the sources. I guess this symlink can be safely removed.