Fix default icon location in 'wrapFirefox'

This pull request probably fixes #2897

*/nix/store/3r83139gsh0qqifv9x7x1fq7h2maxyhg-firefox-with-plugins-31.0/share/applications/firefox.desktop:*
```
[Desktop Entry]
Type=Application
Exec=firefox %U
Icon=/nix/store/7yrxr3yd4c3vgx48c89xi20hybcmdy6n-firefox-31.0/lib/firefox-31.0/icons/mozicon128.png
[...]
```

The icon path doesn't exist. Instead, this is the correct path:
```
/nix/store/7yrxr3yd4c3vgx48c89xi20hybcmdy6n-firefox-31.0/lib/firefox-31.0/browser/icons/mozicon128.png
```

However, I could not test the new firefoxWrapper package because I can't build the new one. The hash of the build inputs are the same, so calling ```nix-build $NIXREPO -A firefoxWrapper``` use the build cache. Is there a way to force a rebuild? or is this a "bug" ?