On attempting to access any web page Firefox brings “Server not found”. Other browsers (Chromium, Opera) have no problems. I tried deleting my .cache/mozilla and .mozilla/firefox directories but the problem persists.
I tried safe mode but it made no difference. I even set up a test user and called Firefox from there, with a completely new (empty - no extensions, etc.) environment but it also made no difference.
At about the time the problem started, thunderbird stopped accessing my mail server, so it seems to be a problem with the way the Mozilla environment handles DNS access. I suspect that a system-software update changed something which Mozilla didn’t like, but I’ve no idea what it could be.
Oh well, if I don’t find a solution I’ll just have to keep using chromium, but firefox has some nice features which aren’t available in chromium …