Firefox Installation / Upgrade

How do I upgrade firefox (1.0.7) installed by default by Ubuntu Install disk to the latest Firefox (1.5), Since the 1.0.7 Version does not allow to install some extensions any more. Either you have de-grade your version to a 1.0 or you have to up-grade your version to 1.5 (Which Ubuntu does not allow me to). Any help that works is appreciated. Thank You and excuse me for Capitalization and Spellings ( I know I am working on them too. )

please have a look at this