Several malicious websites are displaying bogus alerts. In a blog post, Symantec's Parveen Vashishtha explains:
Regardless of what browser is used, the user is presented with the same misleading dialog box that seemingly forces the download of Firefox and Chrome updates.
Clicking the cancel button doesn't seem to do any good, as the misleading dialog box continues to pop up. Vashishtha goes on to say:
If more savvy users don't download the misleading application executable, then these websites will redirect users to a website that, in turn, further redirects to a malicious website that is hosting the infamous Phoenix exploit kit.
Windows PCs that are up to date with bug patches are immune from the exploit kit. As always, users are advised not to click on unverified hyperlinks.