Consider making a portable FireFox button like those on the FireFox affiliates homepage? http://www.spreadfirefox.com/?q=affiliates/homepage People could copy the html and easily link to your site. After looking at one of these blank buttons I imagined one that looked a little more like a usb stick that said Portable FireFox on it - Just a thought. By the way - you need a "this site is best viewed in FireFox 1.5" disclaimer. ;p
The button would need to be a modified version of the Firefox logo to fit that small a size, which means I'd need permission from Mozilla, which could take a while.
As for best viewed with, I prefer designing/developing for every browser where possible. As such, this site actually works with IE5 (with minor visual issues), IE5.5, IE6, Firefox/Mozilla Suite, Safari, Opera, Konqueror, Netscape 6/7, etc. An unstyled but perfectly usable version is sent to IE4, Netscape 4, etc.
Sometimes, the impossible can become possible, if you're awesome!
yeah I meant the disclaimer as a joke but people would probably not take it that way.
Yeah, and besides, the Firefox link buttons on the bottom of the PortableApps.com pages are enough of an indication that the author recommends Firefox, anyway.