Firefox 4 is not longer available for PPC Macs such as the one you have. You can downgrade to Firefox 3.6 from this page. If you would like to, you can download a different build of Firefox not made by Mozilla at http://www.floodgap.com/software/tenfourfox/ but please be aware that for the latter option, the alternate build of Firefox is not supported here.

I apologize for the inconvenience.

I do not have a PPC Mac. My Imac has a 2 GHz Intel Core 2 Duo processor, but I am getting the same message. Is there something else going on?

http://www.mozilla.com/en-US/firefox/4.0/system-requirements/

Your User Agent (sort of a Firefox/system identifier) says that you are on a PPC mac.

Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10_5_8; en-us) AppleWebKit/533.20.25 (KHTML, like Gecko) Version/5.0.4 Safari/533.20.27

Thanks for the link to the system requirements, edmeister. However, my system appears to fit every requirements listed, but still gives the same error message.

Here is what my system profiler tells me about my Imac:

  Mac OS X v. 10.5.8
  Processor: 2 GHz Intel Core 2 Duo, which Wikipedia tells me is "x86"      processor
  3 GB 667 MHz DDR2 SDRAM 

My Activity Monitor says:

  1.87 GB of my system memory free
  303 GB of hard disk space free

Is there some explanation why I am being told Firefox 4 is not being supported on this architecture?

I tried again today, and now was able to replace my old Firefox with version 4, no messages about unsupported architechture. So, if you have an Imac with Intel chip like mine, try again; you might get lucky.

I was able to downgrade. Now how to I retrieve all of my bookmarks and "sign in" information?