hi, this is just a guess, but in case you have nvidia's network access manager or forceware network manager software installed, please go into your system control panel and uninstall it (or go there and have look if it is listed there, if you're unsure about this).

Thanks for the reply. Neither of my PCs have the software you mention. Both PCs recovered Firefox functionality later in the day. My conclusion is that there was some kind of problem on Firefox's servers. That's the only explanation I can think of.