Can you verify you are running OSX 10.0 by clicking the apple icon and choosing about this mac.
Can you tell us what version your current copy of Safari is as the oldest version of Safari that is available is 1.0 which you can get from here, however it is supported by OSX 10.2 soI amnot sure if it will work with OSX 10.0.2 but you may wish to try to download it and see if it will mount for you http://mac.oldapps.com/safari.php
The only version of Firefox for OSX 10.0 is Forefox version .8 which you can get from here http://mac.oldapps.com/firefox.php
Let me know if neither of those work for you you may have to upgrade your version of OSX if the machine will support a newer browser, If this is the case can you tell me the model of your Mac.
Can you click the apple icon then choose about this mac and let me know what processor and how much memory you have so I can check.
Does it give you a processor speed?
In that case as your machine stands you should be able to run OSX 10.3 Panther which you can get from a supplier like We Love Macs http://www.welovemacs.com/6914841a.html
Once you have the disks it is an easy process to insert the disks and then perform an upgrade or archive and install using the instructions included.
With something like 10.3 you will be able to run a much newer version of Safari or Firefox plus there is a lot more of the newer software your machine will support.
Take a look and let me know if you need more info
If you purchase the OSX 10.3 disks it will walk you through the process to upgrade your current OSX version which will automatically remove the oldOSX version so you should not have to do anything other than just follow the upgrade instructions.
If however you have no files that you need to keep the better thing would be to just perform a fresh erase and install, however againthe OSX 10.3 installer will just give you the option to select that choice and it will do the rest for you automatically.
Let me know if you need more info
If you have any other concerns or questions or need any help when you get the disk just let me know