You can do this fairly easily with javascript. Here is an example. If you paste this into the header of a module, it will drop it's text just ahead of the module. I found this example
here.
< script type="text/javascript" >
function detectBrowser()
{
var browser=navigator.appName;
var b_version=navigator.appVersion;
var version=parseFloat(b_version);
if ((browser=="Netscape"||browser=="Microsoft Internet Explorer")
&& (version>=4))
{
alert("Your browser is good enough!");
}
else
{
alert("It's time to upgrade your browser!");
}
}
< /script >
To make this useful, you could use it with some text in a Text/HTML module. Style the block of text to display:none. Then, modify the javascript to change the style to visible if a certain browser is detected.
If you need help in modifying this, the tutorials at w3schools.com ought to be enough. If not, I can flesh this out for you.