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Suddenly Firefox-ESR and Iceweasel can not lookup DNS but /etc/hosts works fine. Other programs does also work fine: konqueror, wget and curl.

I have gone through http://mzl.la/1xKrMnN Firefox can't load websites but other browsers can, no firewalls prevent Firefox from DNS, IPv6 disabled, no proxy and no DNS prefetching.

$ egrep "network.dns.dis|proxy" ~/.mozilla/firefox/*.default/prefs.js 
user_pref("network.dns.disableIPv6", true);
user_pref("network.dns.disablePrefetch", true);
user_pref("network.proxy.type", 0);

I have tried reboot, reinstall, purge & install. Login as a new defined user does not work either. Then:

mkdir debug ; cd debug
strace -o f -ff firefox http://example.org/
grep -rn example.org f*

and to have something to compare with

strace -o k -ff konqueror http://example.org/
grep -rn example.org k*

I have gone through the files but have not found any smoking gun yet. Where to go from now? Reinstall the computer?

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Your size of /etc/hosts is way too big:

$ ls -l /etc/hosts
-rw-r--r-- 1 root root 5128 Oct  2 12:20 /etc/hosts

There is a bug here. If the file size is 1562 bytes Firefox can read it. If it is 1588 bytes or more Firefox can not use DNS.

Conclusion: Either don't use Firefox or cut down /etc/hosts to 1562 bytes.

This bug applies to Firefox version 45.4.0.

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