Firefox Loses DNS in F10

[Richard Heck <rgheck comcast net>]

I've been seeing a weird problem on F10 in Firefox on a fresh install 
onto my daughter's machine. Frequently, after using Firefox for a bit, 
it seems to lose its ability to do DNS lookups. Every URL reports not 
found. Shutting it down and reopening solves the problem. So it's just 
Firefox, not the whole system. Any ideas?

rh