None of the following measures result in a German language interface: Setting general.useragent.locale to "de" or "de_DE.UTF-8" Starting firefox from the command line with "firefox-bin -UILocale de" or "firefox-bin -UILocale de_DE.UTF-8" Starting firefox from the command line with "LC_COLLATE="de_DE.UTF-8" firefox-bin" Using the Quick Locale Switcher extension I have LINGUAS="de en fr nb nb_NO en_GB en_CA", /etc/env.d/02locale is set LANG="de_DE.UTF-8". In firefox, intl.locale.matchOS is set "true"; tests with this set "false" also fail to produce the desired results. All other localization works (including for mail-client/thunderbird-bin-10.0.10). Reproducible: Always Steps to Reproduce: 1. Set LANG="de_DE.UTF-8" in /etc/env.d/02locale 2. Set LINGUAS="de en fr nb nb_NO en_GB en_CA" 3. emerge firefox-bin-17.0.1 4. Start firefox 5. Set in intl.locale.matchOS to "true" in firefox 6. Set general.useragent.locale to "de_DE.UTF-8" 7. Restart firefox Actual Results: The interfaces will be in English. Expected Results: The interfaces should be in German. $ emerge --info =www-client/firefox-bin-17.0.1 Portage 2.1.11.31 (default/linux/amd64/10.0, gcc-4.5.4, glibc-2.15-r3, 3.4.9-gentoo x86_64) ================================================================= System Settings ================================================================= System uname: Linux-3.4.9-gentoo-x86_64-Intel-R-_Core-TM-2_Duo_CPU_T9400_@_2.53GHz-with-gentoo-2.1 Timestamp of tree: Thu, 20 Dec 2012 13:30:01 +0000 ld GNU ld (GNU Binutils) 2.22 app-shells/bash: 4.2_p37 dev-java/java-config: 2.1.11-r3 dev-lang/python: 2.7.3-r2, 3.2.3 dev-util/cmake: 2.8.9 dev-util/pkgconfig: 0.27.1 sys-apps/baselayout: 2.1-r1 sys-apps/openrc: 0.11.5 sys-apps/sandbox: 2.5 sys-devel/autoconf: 2.13, 2.68 sys-devel/automake: 1.9.6-r3, 1.11.6 sys-devel/binutils: 2.22-r1 sys-devel/gcc: 4.5.4 sys-devel/gcc-config: 1.7.3 sys-devel/libtool: 2.4-r1 sys-devel/make: 3.82-r3 sys-kernel/linux-headers: 3.6 (virtual/os-headers) sys-libs/glibc: 2.15-r3 Repositories: gentoo science kde ACCEPT_KEYWORDS="amd64" ACCEPT_LICENSE="* -@EULA" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-O2 -march=native -pipe" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/share/config /usr/share/gnupg/qualified.txt /var/lib/hsqldb" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/env.d /etc/env.d/java/ /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo" CXXFLAGS="-O2 -march=native -pipe" DISTDIR="/usr/portage/distfiles" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync news parallel-fetch protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="ftp://ftp.wh2.tu-dresden.de/pub/mirrors/gentoo ftp://ftp.uni-erlangen.de/pub/mirrors/gentoo http://ftp.uni-erlangen.de/pub/mirrors/gentoo ftp://ftp-stud.hs-esslingen.de/pub/Mirrors/gentoo/ rsync://ftp-stud.hs-esslingen.de/gentoo/ http://ftp-stud.hs-esslingen.de/pub/Mirrors/gentoo/" LANG="de_DE.UTF-8" LDFLAGS="-Wl,-O1 -Wl,--as-needed" MAKEOPTS="-j3" PKGDIR="/usr/portage/packages" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages" PORTAGE_TMPDIR="/var/tmp" PORTDIR="/usr/portage" PORTDIR_OVERLAY="/var/lib/layman/science /var/lib/layman/kde" SYNC="rsync://rsync.gentoo.org/gentoo-portage" USE="X aac acl acpi alsa amd64 amr bash-completion berkdb bzip2 cairo cli consolekit cracklib crypt cups cxx dbus device-mapper dri encode exif extras faac flac fortran gdbm gimp gpm hwdb iconv icu ipv6 jpeg kde laptop lcms lm_sensors mmx modules mp3 mudflap multilib ncurses nls nptl nsplugin opengl openmp pam parport pcre pdf perl png policykit pppd python qt3support readline rtmp semantic-desktop session sqlite sse sse2 sse3 ssl ssse3 startup-notification taglib tcpd theora threads tiff truetype udev unicode usb v4l video vorbis x264 xml zlib" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" ALSA_PCM_PLUGINS="adpcm alaw asym copy dmix dshare dsnoop empty extplug file hooks iec958 ioplug ladspa lfloat linear meter mmap_emul mulaw multi null plug rate route share shm softvol" APACHE2_MODULES="authn_core authz_core socache_shmcb unixd actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias" CALLIGRA_FEATURES="kexi words flow plan sheets stage tables krita karbon braindump" CAMERAS="ptp2" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf superstar2 timing tsip tripmate tnt ubx" INPUT_DEVICES="evdev keyboard mouse synaptics" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LINGUAS="de en fr nb nb_NO en_GB en_CA" PHP_TARGETS="php5-3" PYTHON_SINGLE_TARGET="python2_7" PYTHON_TARGETS="python2_7 python3_2" RUBY_TARGETS="ruby18 ruby19" USERLAND="GNU" VIDEO_CARDS="intel vesa radeon fglrx" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account" Unset: CPPFLAGS, CTARGET, EMERGE_DEFAULT_OPTS, INSTALL_MASK, LC_ALL, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS, USE_PYTHON ================================================================= Package Settings ================================================================= www-client/firefox-bin-17.0.1 was built with the following: USE="(multilib) startup-notification" LINGUAS="de en_GB fr nb_NO -af -ak -ar -as -ast -be -bg -bn_BD -bn_IN -br -bs -ca -cs -csb -cy -da -el -en_ZA -eo -es_AR -es_CL -es_ES -es_MX -et -eu -fa -fi -fy_NL -ga_IE -gd -gl -gu_IN -he -hi_IN -hr -hu -hy_AM -id -is -it -ja -kk -kn -ko -ku -lg -lt -lv -mai -mk -ml -mr -nl -nn_NO -nso -or -pa_IN -pl -pt_BR -pt_PT -rm -ro -ru -si -sk -sl -son -sq -sr -sv_SE -ta -ta_LK -te -th -tr -uk -vi -zh_CN -zh_TW -zu"
After I posted this bug, I discovered the source of the problem. Firefox disabled the non-English language extensions by default after installation; re-enabling those results in the expected user interface.