"Architecture not supported" means that you are trying to install Intel Mac only compatible software on a PPC (Power PC) Mac.
Even though software says that it is OS X or OS 10.5.8 compatible, you must still check the processor requirements.
Example, Opera 11 and 12 are Intel Mac only, but Opera 10 is PPC Mac compatible:
http://www.opera.com/browser/download/requirements/
Firefox 12 states:
- Mac OS X 10.5
- Mac OS X 10.6
- Mac OS X 10.7
- Macintosh computer with an Intel x86 processor
- 512 MB of RAM
- 200 MB hard drive space
PPC Macs, however, don't have Intel x86 processors, so the software is incompatible.
A Mozilla like browser is TenfourFox:
http://osxdaily.com/2011/05/10/run-firefox-4-on-a-powerpc-mac-with-tenfourfox/
You simply need to be more thorough in checking system requirements.