What are alternative browsers for OS 10.3.9? Where find download/install files?
"Upgrade" is going to be the standard answer. Your computer is over 10 years old now, has a slower processor and less memory than my phone, and is well into the timeframe where it can be expected to stop working without notice. If you still have the original disk drive, it is now at twice it's expected service life. But of course you have a backup.

Most software vendors do have old-version archives, but you have to dig for them. Most websites do not maintain compatible versions - mine certainly do not, I cut off at IE9 and I do not test older versions of Chrome, Firefox etc. I'm a bit surprised this site runs on Firefox 2.

You can get a brand-new Air or iMac for $1000. For twice that you can put a 27 inch model on your desk, either one running the latest of everything. And you're good for another decade.