This is difference between the default preference of Firefox 18 beta3 and one of 17.0.1.

Default pref Javascript files (Firefox 18.0.2 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
Default pref Javascript files (Firefox 19 for linux):
(omni.ja){/greprefs.js,/defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js,services-sync.js,services-aitc.js}}
PrefFirefox 18.0.2Firefox 19
browser.download.panel.firstSessionCompletedundefinedfalse
browser.download.panel.shownundefinedfalse
devtools.debugger.chrome-enabledfalsetrue
devtools.debugger.ui.panes-visible-on-startupundefinedfalse
devtools.debugger.ui.remote-win.height400undefined
devtools.debugger.ui.remote-win.width900undefined
devtools.debugger.ui.stackframes-pane-visibletrueundefined
devtools.debugger.ui.variables-non-enum-visibleundefinedtrue
devtools.debugger.ui.variables-pane-visibletrueundefined
devtools.debugger.ui.variables-searchbox-visibleundefinedfalse
devtools.debugger.ui.variables-sorting-enabledundefinedtrue
devtools.debugger.ui.win-heightundefined400
devtools.debugger.ui.win-widthundefined900
devtools.debugger.ui.win-xundefined0
devtools.debugger.ui.win-yundefined0
dom.new_bindingstrueundefined
dom.placeholder.show_on_focusundefinedtrue
font.size.inflation.maxRatioundefined0
gfx.canvas.azure.enabledtrueundefined
gfx.content.azure.enabledundefinedfalse
gfx.font_rendering.harfbuzz.scripts7-1
image.high_quality_downscaling.enabledfalsetrue
image.mem.decode_bytes_at_a_time409616384
intl.charsetmenu.browser.more1ISO-8859-1, ISO-8859-15, IBM850, x-mac-roman, windows-1252, ISO-8859-14, ISO-8859-7, x-mac-greek, windows-1253, x-mac-icelandic, ISO-8859-10, ISO-8859-3ISO-8859-1, ISO-8859-15, IBM850, macintosh, windows-1252, ISO-8859-14, ISO-8859-7, x-mac-greek, windows-1253, x-mac-icelandic, ISO-8859-10, ISO-8859-3
intl.charsetmenu.browser.more3GB2312, gbk, gb18030, HZ-GB-2312, ISO-2022-CN, Big5, Big5-HKSCS, x-euc-tw, EUC-JP, ISO-2022-JP, Shift_JIS, EUC-KR, x-windows-949, x-johab, ISO-2022-KRGB2312, gbk, gb18030, HZ-GB-2312, ISO-2022-CN, Big5, Big5-HKSCS, x-euc-tw, EUC-JP, ISO-2022-JP, Shift_JIS, EUC-KR, x-johab, ISO-2022-KR
intl.charsetmenu.browser.more5ISO-8859-6, windows-1256, IBM864, ISO-8859-8-I, windows-1255, ISO-8859-8, IBM862ISO-8859-6, windows-1256, ISO-8859-8-I, windows-1255, ISO-8859-8, IBM862
javascript.options.relimittrueundefined
layout.css.prefixes.animationsundefinedtrue
layout.css.prefixes.border-imageundefinedtrue
layout.css.prefixes.transformsundefinedtrue
layout.css.prefixes.transitionsundefinedtrue
network.disable.ipc.securityundefinedfalse
network.http.spdy.ping-threshold4458
network.stricttransportsecurity.preloadlistundefinedtrue
pdfjs.disabledtruefalse
security.ssl3.dhe_dss_des_shafalseundefined
security.ssl3.dhe_rsa_des_shafalseundefined
security.ssl3.ecdh_ecdsa_null_shafalseundefined
security.ssl3.ecdh_rsa_null_shafalseundefined
security.ssl3.ecdhe_ecdsa_null_shafalseundefined
security.ssl3.ecdhe_rsa_null_shafalseundefined
security.ssl3.rsa_1024_des_cbc_shafalseundefined
security.ssl3.rsa_1024_rc4_56_shafalseundefined
security.ssl3.rsa_des_shafalseundefined
security.ssl3.rsa_fips_des_shafalseundefined
security.ssl3.rsa_null_md5falseundefined
security.ssl3.rsa_null_shafalseundefined
security.ssl3.rsa_rc2_40_md5falseundefined
security.ssl3.rsa_rc4_40_md5falseundefined
security.warn_entering_securefalseundefined
security.warn_entering_secure.show_oncefalseundefined
security.warn_entering_weaktrueundefined
security.warn_entering_weak.show_oncetrueundefined
security.warn_leaving_securefalseundefined
security.warn_leaving_secure.show_oncefalseundefined
security.warn_submit_insecurefalseundefined
security.warn_submit_insecure.show_oncefalseundefined
security.warn_viewing_mixedtrueundefined
security.warn_viewing_mixed.show_oncetrueundefined
services.sync.log.logger.userapiundefinedDebug
services.sync.prefs.sync.security.warn_entering_securetrueundefined
services.sync.prefs.sync.security.warn_entering_weaktrueundefined
services.sync.prefs.sync.security.warn_leaving_securetrueundefined
services.sync.prefs.sync.security.warn_submit_insecuretrueundefined
services.sync.prefs.sync.security.warn_viewing_mixedtrueundefined
toolkit.scrollbox.horizontalScrollDistanceundefined5
ui.touch_activation.delay_msundefined50
webgl.can-lose-context-in-foregroundundefinedtrue