For instance, the WiSIG (Wireless Sensing Interest Group) site displays this message if you visit using Firefox:
Thanks for visiting www.wisig.org. This site as well as SensorsKTN.com are best viewed in Internet Explorer 5.5 or above.
If you have a copy of Internet Explorer, now's the time to fire it up and try again ...
Okay, so it's best viewed in IE (according to them), but so what? Are they saying I won't even be able to see the text if I use some other browser? Is the threat to my mental health from the mis-rendered site so bad that I'm not even allowed to try unless I resort to a browser that doesn't even deserve to exist on my system, let alone be "fired up" at the whim of random sites?
Seriously guys, specifically the webmasters among you, even if you "optimise" a site for a specific browser you can still make it work right in other browsers, or failing that at least make it gracefully degrade, or failing that at least let people see it. If nothing else the text will still be visible and they will be able to get the information they want. Forcing the use of a specific browser is very rude behaviour.
If anyone from the WiSIG reads this then sorry for picking on your site but it was one I was trying to visit earlier and I usually expect technology based sites to represent themselves well on the internet. Have a word with your webmaster and see what he/she/it can do for you.