Firefox suggests you download libstdc++5 to install an icon on
the desktop to start FF5. That costs 1 Mb, and doesn't work for me.
Luckily just add a 'new blank file' to the LXDE desktop, for example
a file called ff.sh, executable, contents you guessed:
/etc/local/firefox/firefox. Acts like a click-on icon.
Pretty plain, but it works, costs only a few bytes.

FF5 is pretty snappy; I think I might switch.