Hi Chris,
The latest version of the 10.0.x ESR series was 10.0.12, released in
January 2013. This version is not supported any more and has known
security bugs. 17.0.x ESR series is currently being phased out (but
still supported for two more releases, i.e. ~ 2x6 weeks). Switching to
the new 24.0.x ESR releases is highly recommended if you can do it
(otherwise browsing to untrusted sites is not a good idea).
Unfortunately there seem to be no builds available for the ESR XULRunner
releases, but you can build it yourself from source if you need to. The
source is here:
http://hg.mozilla.org/releases/mozilla-esr10
Philipp