Codec Error with Firefox 4

I've been searching this site and the web for days and can't find my problem.

I'm running 10.5.8 on an 2.16 Intel Core 2 Duo iMac. I downloaded the dmg file for firefox 4 and when I try to run it I get a "Failed To Mount" error, and it says "codec overrun".

I don't really know what this means, but after some searching I've repaired disc permissions in Disk Utilities, I've reinstalled the dmg file thinking maybe it was corrupt, I don't know what's going on. In all my searches online the only thing I'm finding is that it's not compatible with non-Intel based Macs.

Are you using a download manager such as Speed Download? If so, uninstall it properly using the included uninstaller. It causes more problems than it solves.

I ask because they frequently download files in chunks, putting them back together when the download is complete. Doing this may mess up the checksum (this checks to make sure that the file is valid).

I've also read that such issues can be related to a router firewall but let's start with the first question.

No, no speed downloader.

I do have a router, but it's same router that I've used for the past 3 years with no issues.
The only difference is about a week ago I moved and now I'm using it wirelessly, before I was always plugged in.

Anyone?

Bueller?

Bueller?

Trash the .dmg file including any in Downloads Folder and download a new file from here and see how that goes. Codec troubles sometimes come about with torrent downloads if you have provided your password:-

https://www.mozilla.com/en-US/firefox/system-requirements.html