Firefox is a browser; do you mean that your MacBook Pro currently has Mac OS X 10.5.8 (“Leopard”) as its operating system?
If you can’t afford to lose your documents, have you been making regular backups of your MacBook Pro’s internal disk to date?
You can upgrade your MacBook Pro to at least Snow Leopard (Mac OS X 10.6.8). Depending upon which model you have, you could potentially upgrade it to the current version, Mavericks (OS X 10.9.2), but you’ll need to get to 10.6.8 before you can upgrade any further. You can do that by purchasing the white retail Snow Leopard DVD, installing Snow Leopard, and then running Software Update to get to 10.6.8. Note that if you currently depend upon any PowerPC applications, they will not work on versions of OS X later than 10.6.8. You might also need to upgrade its installed RAM to ensure good performance on a newer operating system.
Before you do any operating system upgrade, be sure that you have made a backup of your valuable documents.
Note that your current freeze up could be resolvable without having to upgrade your operating system. Would you prefer to stay with Leopard and resolve the freeze, or would you prefer to upgrade to a newer operating system?