For change in Useragent string of Mozilla browser:
Just get the IP of your client and add as in the User agent string as described below.:
How to edit the User Agent string
To change the User Agent string, just enter about:config
as an address in the
address bar of FireFox, the location where you normally enter a URL (link). I recommend
to preserve the original value, which you can get when you enter just about:
in the address bar.
Now press the right mouse button to get the context menu and select "String" from the menu
entry "New". Enter the preference name "general.useragent.override", without the quotes. Next,
enter the new User Agent value you want Mozilla Firefox to use.
second way:
Create one .Net application with browser control which opens your websites.
which also add mac ID in the header. So you don't need to add it all your client PC manually.
Here i am providing two article for reference
1) How to get MAC ID in c#
http://stackoverflow.com/questions/850650/reliable-method-to-get-machines-mac-address-in-c-sharp
2) Just get Mac id and add it to the user agent string of browser control .
http://stackoverflow.com/questions/937573/changing-the-useragent-of-the-webbrowser-control-winforms-c-sharp