I'm currently running Firefox 34.0 and Chrome 39.0 on Ubuntu 14.04.
Firefox is presenting with extremely slow DNS look-ups that aren't occurring in Chrome. In order to repro, I've shut down both Ffox and Chrome, and opened new windows. I then visit a new domain that I haven't yet visited, first in Ffox, then Chrome. In order to make sure it's not caching causing the difference, I then try a different domain first with Chrome, and then with Ffox. I repeated this a number of times to make sure.
When visiting a new domain for the first time with Ffox, I see the "Looking up domainname" dialog, and it takes roughly 10 seconds in order to complete the page load, mostly due to what seems to an extremely slow DNS lookup. I'm having no such issues in Chrome.
Additionally, after the first time I visit a website in Ffox, future pageloads are snappy, presumably due to some sort of caching. However, closing Ffox and opening it again will seemingly clear that cache.
I have tried a couple of things here and here, though neither has resolved the issue.
I feel as though it would be very useful to see the requests that Ffox is making in order to understand what's going on, but I've never done that sort of thing before. What would be some useful tools/and how would I use them to diagnose a problem like this?
sudo /etc/init.d/nscd restart
to flush that cache in Ubuntu - see How to flush dns in ubuntu 12.04?