Firefox and Thunderbird cannot resolve host names - anything else can

Question

One night my firefox just started showing "Server not found" for all websites (i.e. "Firefox can't find the server at www.mozilla.com."). I noticed that it can find websites if make an entry in the /etc/hosts-file.

I have already started firefox in safe mode, the problem persisted.

I removed my ~/.mozilla folder entirely, didn't help either.

At this point I noticed that thunderbird was having the same problem and assumed it must be a shared library. I'm running gentoo linux, where the packages are distributes as source code, if available, so I started rebuilding each and every package on my system ("emerge -av --emptytree world"), still the same error.

I then tried installing the source-package of firefox (I was using the binary package "firefox-bin"), but that still didn't help.

Does anyone have an idea what I might do next?

EDIT (Thx, Gilles)

Installed Konqueror and verified that it doesn't have this issue (running fluxbox.)

I have manually set the proxy settings of firefox to "No Proxy", no change.

I have disabled ipv6 in about:config, still the same.

Answer 1

Check your proxy settings in “Edit / Preferences / Network”. Maybe Firefox and Thunderbird are using a Freedesktop proxy setting which other programs you've tried aren't, and there's a problem with that proxy. Can other Freedesktop programs (e.g. Konqueror, Nautilus, …) resolve host names?

Another possibility is that Firefox is now checking IPv6 names first, but you don't get DNS for IPv6. Check the state of preferences containing ipv6 in about:config. (I don't know why it wouldn't fall back to IPv4 though.)

Answer 2

The problem is finally solved. My /etc/hosts file was quite large, containing many comments (2.7Kb) and it looks like firefox was having trouble with that. Reducing the file to a reasonable amount solved the problem!