Forgot to add that Firefox no longer supports the PPC platform, but an alternative called TenFourFox is available.
You can download TenFourFoxG5 from here:
http://www.floodgap.com/software/tenfourfox/
Now for the interesting bit: TenFourFox, like Firefox, does not support most plug-ins. Unless you are prone to headaches (in which case don't) you can read about that here:
http://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported
But there is a workaround!
Open TenFourFox and type in the address bar: about:config (no spaces) and click return. This gives you a warning that it might harm the application. Ignore that and click on 'I'll be careful, I promise' and you get the config file that you can edit - with great care.
Look/search (scroll down) for: tenfourfox.plugins.enabled
Set it to true. (Double click it to toggle it)
Now close TenFourFox, open it again, and Flash will now work, as it now uses the plug-ins that Safari has stored.
I am using Flash 11.5.502.55 in TenFourFox and it works just fine.