Starting here on the assumption that this is a browser function.

Firefox 3.5.3 on ArchLinux

I just tried downloading a free trial of Acrobat 9 to see if I can get it going on CrossOver. The Adobe site will not download because it sees I am not using Windows.

I searched in the detailed about:config file and found nothing that looked relevant.

So: How does FF (or any broswer) tell a site what OS is being used?

It's a JavaScript call, you can use Opera of Konqueror, they have some spoofing options.

OK--now I'm interested: JavaScript call to what?

(I will try the other browsers...)

I'm confused. And, back on topic: I thought that this was where all that user-agent junk came into play.. For stuff like this? And, FF has the same spoofing options as other browsers, doesn't it?

As far as I know, the various browser-builds for the various OS types simply specify the OS-identification string that they are to present to anyone who asks.

The source-code of a major piece of software (such as a web browser) is designed in such a way that various parts of the source-code will or will not be included, depending on various build-time options that have been selected (including "the type of OS that this is being built for"). Through mechanisms such as these, the appropriate OS-identifier is constructed at compile time.

A site such as Adobe's can look at the browser identification string (which is presented as part of the HTTP header) to base its decision.

There's a twist, of course: many browsers (such as Konqueror or Opera) allow you to specify the string that will be sent to the host. You can, for example, "pretend to be Internet Explorer running on a Windows box." The host has no way to know that you are lying.

Set the user agent string to whatever you like: most decent browsers let you.

Evo2.

The browser has a string called the 'User Agent'. This can be read by javascript, or by code on the server.
It's generally possible to configure the User Agent string - you can make the browser 'lie' about its identity. For Firefox, you will want an extension, "User Agent Switcher" to do this.

Its in the user.agent
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7

The User Agent Switcher extension for Firefox lets you change it to whatever.
https://addons.mozilla.org/en-US/firefox/addon/59

Thanks--everyone!!

Somehow I had thought you could do this with about:config... Now I have the plug-in and it seems to do the trick.