'Twas brillig, and the slithy toves did gyre and gimble in the wabe

Of course if you used a modern browser (IE8 for you, 9 also has it) then you would have this tool built in. Under the development tools (F12) you can select to spoof a selection of browsers including FF and Chrome.

Not that I think sites should block based on user agent, Chrome's experiments page did this an age ago, as did the Safari tests... only reason it annoyed me was because actually IE9 frame for some time was beating the pants off the pair of them. Not so much now...