how can I do an upgrade to safari (and firefox) for mac os x 10.5.8

There are no upgrades for PPC versions of Leopard. Firefox is not an Apple product. Get updates from their site.

The highest version of FireFox that wil run on a PowerPC Mac is v3.something. The current version of FF is v26. No joy there.

TenFourFox is a Mac port of a later version of FF and is designed for PowerPC Macs. It is your best browser option today.

http://www.floodgap.com/software/tenfourfox/

The highest version of Safari that will run on a PwoerPC MAc with 10.5.8 is 5.0.6

There is a fair version of iCab that runs under demo status, with no faults for free, so you pay if you like it. The last update was 5.1.1 for PPC as far as my version tells me. iCab: http://www.icab.de/dl.php

TenFourFox is running version 24.2 now, and if you don't care for the default tabs, there is an add-on from Mozilla to cure it. Add-on: 'Hide Tab Bar with One Tab' is a beta, & gets rid of a few pesky Tab issues; so v 24.2 can then act like 17.0.11 at least in how Tabs are handled.

And SeaMonkey-PPC has a recent edition that may be OK, but I found the changes over the last stable version make the CPU usage go to near 100% & fans run hard, with only one browser window and no other apps; that's 2.22.1. So I reverted to 2.13 from nearly a year ago as it is best, and emailed the developer about this matter. At some point, there was a change and this makes the fan(s) run and CPU usage go to max... I tried a few and then settled for a version I already had in private archive.

seamonkey-ppc: http://code.google.com/p/seamonkey-ppc/downloads/list

The recent TenFourFox changes a few things (from v. 17.0.11) in that your options to see youtube or other content have been narrowed. Content that used to be able to function if you did a work-around won't now. Not certain if the quicktime enabler for 10.4.fx still works in the new browser. Read carefully.

Good luck & happy computing! 🙂