To reproduce:
apk add firefox icedtea-web-mozilla icedtea-web

Open a java applet demo:

file:///usr/lib/jvm/java-1.6-openjdk/demo/applets/ArcTest/example1.html

result is:

$ firefox 
java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11) (Alpine 1.6.0_p24-r0)
OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007fbcd78358e7, pid=12136, tid=140449049483072
#
# JRE version: 6.0_24-b24
# Java VM: OpenJDK 64-Bit Server VM (20.0-b12 mixed mode linux-amd64 compressed oops)
# Derivative: IcedTea6 1.11
# Distribution: Custom build (Sat Feb 11 03:34:14 UTC 2012)
# Problematic frame:
# C  [libnet.so+0x98e7]  NET_SockaddrEqualsInetAddress+0x797
#
# An error report file with more information is saved as:
# /home/ncopa/java/hs_err_pid12136.log
#
# If you would like to submit a bug report, please include
# instructions how to reproduce the bug and visit:
#   http://icedtea.classpath.org/bugzilla
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#
Error: Unable to fetch applet instance id from Java side.

(from redmine: issue id 1037, created on 2012-03-01, closed on 2012-10-03)