Make sure that your anti-virus software isn't corrupting that XPI (ZIP) file because that error indicates that the file has been modified (it is a signed extension with a META-INF folder).

I see a DLL file for Windows (dtTransparency.dll) in that extensions file in the components folder, so I'm not sure if it really works on a Mac. It may also be a problem with your PPC platform.

I am not a sophisticated user, so I do not understand much of your reply (e.g. I do not know what a PPC platform is). However, I can tell you that I am not running any anti-virus software on my eMac, so that type of anti-virus software cannot be corrupting the XPI file. FYI: I am using Airport Extreme for my wireless network. Does that have built-in anti-virus software or use some type of encryption that could be causing me to get this message?

You have an old Mac with a PowerPC processor according your system details list, that is what is meant with PPC platform. That toolbar may not support a PowerPC processor, but that shouldn't cause that error message because the toolbar isn't installed yet. The error that you get indicates that the extensions XPI file has been modified and you will have to try to install it another time.