All that has to happen is to have Firefox open, not even using it, and you go to it and attempt to open a page, by bookmark or hand-typed URL, and it sits there and does NOTHING, doesn't attempt to open the page, it just sits there. Closing and opening it again does not fix the problem, it still just sits there. Finding the process and KILLing it, the process goes away, but re-launching Firefox, it still has the problem, it just sits there when you click a bookmark or enter a URL. I tried uninstalling and reinstalling it. I tried 'Refreshing' it back to it's default, newly-installed state, doesn't solve the problem. The only thing that seems to fix the problem TEMPORARILY is to reboot Ubuntu. I installed Chrome after a while just to see if any browser would work when Firefox starts acting this way, and Chrome is working when Firefox won't. You can open a Terminal window and ping an IP address, so it's not like it's a network problem.

This is a fairly new installation of Ubuntu, started out with v17.x, just upgraded to v18.x, that didn't help either. I'm not having problems with anything else in Ubuntu. Note the Firefox came with Ubuntu by default, it's not like I forced it to install or installed it later.