It really depends on what desktop-environment you are using.
If you use KDE then it will have to load some libraries which are not present if all other apps are native KDE apps - since firefox is a GTK application.
If you use Gnome - which is built upon GTK - these things will already be there and not need to be loaded first.
But eighter way - this is only for the first start-up... - after that it is much faster in any environment.
Code:
ldd firefox-bin
linux-gate.so.1 => (0xffffe000)
libmozjs.so => /usr/lib/mozilla-firefox/libmozjs.so (0xb7e95000)
libxpcom.so => /usr/lib/mozilla-firefox/libxpcom.so (0xb7e92000)
libxpcom_core.so => /usr/lib/mozilla-firefox/libxpcom_core.so (0xb7dea000)
libplds4.so.6 => /usr/lib/nspr/libplds4.so.6 (0xb7de7000)
libplc4.so.6 => /usr/lib/nspr/libplc4.so.6 (0xb7de2000)
libnspr4.so.6 => /usr/lib/nspr/libnspr4.so.6 (0xb7da6000)
libpthread.so.0 => /lib/libpthread.so.0 (0xb7d7e000)
libdl.so.2 => /lib/libdl.so.2 (0xb7d7a000)
libgtk-x11-2.0.so.0 => /usr/lib/libgtk-x11-2.0.so.0 (0xb7a78000)
libgdk-x11-2.0.so.0 => /usr/lib/libgdk-x11-2.0.so.0 (0xb79f5000)
libatk-1.0.so.0 => /usr/lib/libatk-1.0.so.0 (0xb79db000)
libgdk_pixbuf-2.0.so.0 => /usr/lib/libgdk_pixbuf-2.0.so.0 (0xb79c4000)
libpangocairo-1.0.so.0 => /usr/lib/libpangocairo-1.0.so.0 (0xb79bc000)
libpango-1.0.so.0 => /usr/lib/libpango-1.0.so.0 (0xb7983000)
libcairo.so.2 => /usr/lib/libcairo.so.2 (0xb7937000)
libgobject-2.0.so.0 => /usr/lib/libgobject-2.0.so.0 (0xb78fd000)
libgmodule-2.0.so.0 => /usr/lib/libgmodule-2.0.so.0 (0xb78f9000)
libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0xb7874000)
libpangoxft-1.0.so.0 => /usr/lib/libpangoxft-1.0.so.0 (0xb786c000)
libpangoft2-1.0.so.0 => /usr/lib/libpangoft2-1.0.so.0 (0xb7846000)
libX11.so.6 => /usr/lib/libX11.so.6 (0xb7777000)
libm.so.6 => /lib/libm.so.6 (0xb7754000)
libstdc++.so.6 => /usr/lib/gcc/i686-pc-linux-gnu/3.4.4/libstdc++.so.6 (0xb7684000)
libgcc_s.so.1 => /usr/lib/gcc/i686-pc-linux-gnu/3.4.4/libgcc_s.so.1 (0xb767b000)
libc.so.6 => /lib/libc.so.6 (0xb7560000)
/lib/ld-linux.so.2 (0xb7f31000)
libXrender.so.1 => /usr/lib/libXrender.so.1 (0xb7558000)
libXrandr.so.2 => /usr/lib/libXrandr.so.2 (0xb7554000)
libXi.so.6 => /usr/lib/libXi.so.6 (0xb754c000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0xb7525000)
libXcursor.so.1 => /usr/lib/libXcursor.so.1 (0xb751b000)
libXext.so.6 => /usr/lib/libXext.so.6 (0xb750c000)
libXfixes.so.3 => /usr/lib/libXfixes.so.3 (0xb7507000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0xb7493000)
libz.so.1 => /lib/libz.so.1 (0xb7481000)
libpng12.so.0 => /usr/lib/libpng12.so.0 (0xb744c000)
libXft.so.2 => /usr/lib/libXft.so.2 (0xb7439000)
libexpat.so.0 => /usr/lib/libexpat.so.0 (0xb7419000)