This is difference between the default preference of Firefox 27.0 beta 1 and one of 26.0.

Default pref Javascript files (Firefox 26.0 for Linux):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}
Default pref Javascript files (Firefox 27.0 beta 1 for Linux):
(omni.ja){/greprefs.js,/defaults/pref/services-sync.js}, (browser/omni.ja)defaults/preferences/{firefox.js,firefox-branding.js,firefox-l10n.js}
PrefFirefox 26.0Firefox 27.0 beta 1
apz.axis_lock_modeundefined0
browser.cache.memory_limitundefined51200
browser.cache.use_new_backendundefined0
browser.safebrowsing.warning.infoURLhttps://www.mozilla.org/%LOCALE%/firefox/phishing-protection/undefined
browser.sessionstore.asyncundefinedtrue
browser.uitour.enabledundefinedfalse
browser.uitour.pinnedTabUrlundefinedhttps://support.mozilla.org/%LOCALE%/kb/pinned-tabs-keep-favorite-websites-open
browser.uitour.themeOriginundefinedhttps://addons.mozilla.org/%LOCALE%/firefox/themes/
browser.uitour.whitelist.add.260undefinedwww.mozilla.org,support.mozilla.org
datareporting.healthreport.service.providerCategorieshealthreport-js-provider-defaulthealthreport-js-provider-default,healthreport-js-provider-beta
devtools.appmanager.manifestEditor.enabledundefinedfalse
devtools.browserconsole.filter.csslogundefinedfalse
devtools.debugger.pretty-print-enabledundefinedtrue
devtools.defaultColorUnitundefinedhex
devtools.responsiveUI.enabledtrueundefined
devtools.scratchpad.enabledtrueundefined
devtools.shadereditor.enabledundefinedfalse
devtools.webconsole.filter.csslogundefinedfalse
dom.datastore.enabledundefinedfalse
dom.mms.defaultServiceIdundefined0
dom.sms.defaultServiceIdundefined0
dom.telephony.defaultServiceIdundefined0
dom.voicemail.defaultServiceIdundefined0
dom.workers.sharedWorkers.enabledundefinedfalse
gfx.content.azure.enabledfalsetrue
image.mem.allow_locking_in_content_processesundefinedtrue
image.mem.surfacecache.max_size_kbundefined102400
image.mem.surfacecache.min_expiration_msundefined60000
image.mem.surfacecache.size_factorundefined64
layout.css.all-shorthand.enabledundefinedtrue
layout.css.text-align-true-value.enabledundefinedfalse
layout.css.unset-value.enabledundefinedtrue
layout.enable_japanese_specific_transformfalseundefined
layout.imagevisibility.enabled_for_browser_elements_onlyundefinedfalse
media.navigator.video.max_frundefined0
media.navigator.video.max_fsundefined0
media.peerconnection.default_iceservers[{"url": "stun:23.21.150.121"}][{"url": "stun:stun.services.mozilla.com"}]
media.peerconnection.trickle_iceundefinedtrue
media.tabstreaming.heightundefined240
media.tabstreaming.time_per_frameundefined40
media.tabstreaming.widthundefined320
network.http.spdy.enabled.v3-1undefinedtrue
network.seer.enable-hover-on-sslundefinedfalse
network.seer.enabledundefinedtrue
network.seer.max-queue-sizeundefined50
network.seer.page-degradation.dayundefined0
network.seer.page-degradation.maxundefined50
network.seer.page-degradation.monthundefined10
network.seer.page-degradation.weekundefined5
network.seer.page-degradation.yearundefined25
network.seer.preconnect-min-confidenceundefined90
network.seer.preresolve-min-confidenceundefined60
network.seer.redirect-likely-confidenceundefined75
network.seer.subresource-degradation.dayundefined1
network.seer.subresource-degradation.maxundefined100
network.seer.subresource-degradation.monthundefined25
network.seer.subresource-degradation.weekundefined10
network.seer.subresource-degradation.yearundefined50
ril.numRadioInterfaces10
security.OCSP.GET.enabledundefinedfalse
security.enable_tls_session_ticketstrueundefined
security.ssl3.dhe_dss_camellia_128_shatruefalse
security.ssl3.dhe_dss_camellia_256_shatruefalse
security.ssl3.dhe_dss_des_ede3_shatrueundefined
security.ssl3.ecdh_ecdsa_aes_128_shatrueundefined
security.ssl3.ecdh_ecdsa_aes_256_shatrueundefined
security.ssl3.ecdh_ecdsa_des_ede3_shatrueundefined
security.ssl3.ecdh_ecdsa_rc4_128_shatrueundefined
security.ssl3.ecdh_rsa_aes_128_shatrueundefined
security.ssl3.ecdh_rsa_aes_256_shatrueundefined
security.ssl3.ecdh_rsa_des_ede3_shatrueundefined
security.ssl3.ecdh_rsa_rc4_128_shatrueundefined
security.ssl3.ecdhe_ecdsa_aes_128_gcm_sha256undefinedtrue
security.ssl3.ecdhe_ecdsa_des_ede3_shatrueundefined
security.ssl3.ecdhe_rsa_aes_128_gcm_sha256undefinedtrue
security.ssl3.rsa_fips_des_ede3_shatruefalse
security.ssl3.rsa_seed_shatruefalse
security.tls.version.max13
signon.SignonFileNamesignons.txtundefined
signon.SignonFileName2signons2.txtundefined
signon.SignonFileName3signons3.txtundefined
snav.enabledundefinedfalse
social.allowMultipleWorkersundefinedtrue
toolkit.asyncshutdown.timeout.crashundefined60000
ui.touch_activation.duration_msundefined100
urlclassifier.download_allow_tableundefinedgoog-downloadwhite-digest256
urlclassifier.download_block_tableundefinedgoog-badbinurl-shavar
urlclassifier.gethashtablesgoog-phish-shavar,goog-malware-shavarundefined
urlclassifier.malware_tableundefinedgoog-malware-shavar
urlclassifier.phish_tableundefinedgoog-phish-shavar