Firefox
Firefox 3.5.9 is included in OLPC OS 10.1.3 and can be activated from GNOME.
Installing Adobe Flash
Firefox on OLPC OS does not include Adobe Flash plugin, and as a result some web sites do not work as designed. To install the plugin see Adobe Flash, in particular the section for Browse.
Sugar
See Firefox/Obsolete for a Sugarised Firefox for old OLPC OS releases prior to 10.x.
Firefox can also run from within Terminal. Type firefox at the Terminal prompt and press enter.