I just recently switched to FF 57.0 after using Chrome and Safari for a long time, and I've noticed that my DNS lookups appear to be extremely slow and unreliable. Specifically, visiting sites will very often give either a long pause with the FF status bar showing "Looking up ..." or an error page with a "can't find the server" message. I'm reasonably certain that it's not actually a server issue because this happens for sites like github.com and mozilla.org which I know are currently up, and I'm reasonably certain that it's not a systemwide issue with my internet because any time this happens in FF, I can always successfully and quickly load the same URL in Chrome. In addition, every single time I get the "can't find the server" error, just trying the same URL a second time results in it loading immediately. I've had this pattern (slow "looking up" followed by "can't find the server", followed by requesting the site again and seeing it load instantly) with the last half-dozen servers I've connected to in FF, and I've never had it in Chrome or Safari. This appears to be a DNS-only issue, since loading subsequent pages from the same server is always fast. It's only when I first connect to a server that I haven't connected to recently that I see the issue. So far, I have tried "refreshing" Firefox, removing all add-ons, clearing the cache, and restarting but to no avail. I'm on OSX 10.11.6 with Firefox 57.0.