I've actually read of a few people doing this. Checking to see what browser the visitor uses then displaying the adsense/firefox link. In the examples I've seen they didn't completely block out the site to IE visitors, they simply added a "toolbar" like alert telling you that their site was designed for firefox.
I don't believe that is against the adsense terms. Completely blocking out the site to IE visitors and forcing them to the link may be though.