I support a major website with millions of users. We bring out new versions of code by hosting it on new infrastructure and at time of cutover we update DNS entries to point to the new infrastructure. Typically it takes 3-4 days for all users to migrate to the new release as it takes time for DNS to propagate to all corners of the internet and some ISPs have cached DNS. But recently we started seeing an unusual pattern, even a week after the cutover we see a small number of users, all running Firefox, still hitting our old site. If this was some ISP or an issue with our DNS servers I would expect multiple browsers being affected, but this was specific to Firefox. I was expecting Firefox to cache a DNS entry for a few minutes, not weeks. Has anyone run into the same issue ? It's adding a lot of extra cost to keep both versions of the site running. Just some stats, a vast majority of the users are experiencing the problem are on FF v26/27.