What is preventing Firefox from opening websites?

I have come across a strange behavior of Firefox on Ubuntu 14.04. I left office with my laptop turned on so I could listen to music. Before I left office, Firefox was working normally. Now I came to library, opened my laptop and Firefox and what happened was that Firefox was unable to open any pages, including Google.com. Firefox was running smoothly, without any delays, it just returned *Server not found* on any page I tried. What I did was, as usual in such cases, restart NetworkManager with command sudo killall NetworkManager which also automatically starts NetworkManager after it. The laptop connected to network, I relaunch Firefox and again nothing. So I ran ps auxw | grep firefox to see if there is some strange Firefox background process running but everything appeared as usual. I am sorry but I didn't take a screenshot, my mistake. I closed Firefox and executed ps auxw | grep firefox again and as expected, there was only one output line like this campovs+ 2715 0.0 0.0 5924 2196 pts/0 S+ 16:43 0:00 grep --color=auto firefox which means that grep was searching for firefox. At this point I was 100% sure there has to be something with the internet connection and not Firefox. So I decided to proceed with regular internet connection checking, that being executing ping google.com Packets were flying which also eliminated possible DNS errors, although I have no idea how those could happen since I haven't done anything on my laptop and the network I was connected to was *eduroam*. I ran another command curl -v google.com and it, as expected, returned HTML code of google.com with last line * Connection #0 to host google.com left intact which basically means everything is ok. I was left out of options and I knew that restarting the laptop will almost certainly solve the problem, although I didn't even know what was causing it. I ran sudo reboot and now Firefox is working. **So my question is, does anyone of you know, what might cause such behavior?**