I got this fixed also on 13.2 with upstream firefox. This is what it spits upon start/stop (not sure if it's helpful): linux-q0c7:/tmp/firefox$ ./firefox (process:8478): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed 1420642385616 GMPInstallManager.simpleCheckAndInstall INFO Last check was: 1021840 seconds ago, minimum seconds: 86400 1420642385616 GMPInstallManager._getURL INFO Using url: https://aus4.mozilla.org/update/3/GMP/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml 1420642385617 GMPInstallManager._getURL INFO Using url (with replacement): https://aus4.mozilla.org/update/3/GMP/34.0.5/20141126041045/Linux_x86_64-gcc3/en-US/release/Linux%203.16.7-7-desktop%20(GTK%202.24.25)/default/default/update.xml 1420642385618 GMPInstallManager.checkForAddons INFO sending request to: https://aus4.mozilla.org/update/3/GMP/34.0.5/20141126041045/Linux_x86_64-gcc3/en-US/release/Linux%203.16.7-7-desktop%20(GTK%202.24.25)/default/default/update.xml 1420642386471 GMPInstallManager.onLoadXML INFO request completed downloading document 1420642386472 GMPInstallManager.onLoadXML INFO allowNonBuiltIn: false 1420642386476 GMPInstallManager.simpleCheckAndInstall INFO Found 1 addons advertised. 1420642386476 GMPInstallManager.simpleCheckAndInstall INFO Found addon: gmp-gmpopenh264 (isValid: true, isInstalled: true, isOpenH264: true, hashFunction: sha512, hashValue: 472543802d4d5fa9144365088c81d4b0457e765e57fa153ced03c41d790007b8351f6ed702d5ed39be66cfdbdb83bcbf89341ae1551190afd29888cd7a5adfb6, size: 389073) 1420642386476 GMPInstallManager.simpleCheckAndInstall INFO No new addons to install, returning console.error: Message: Error: SessionFile is closed Stack: SessionFileInternal.write@resource://app/modules/sessionstore/SessionFile.jsm:268:29 this.SessionFile.write@resource://app/modules/sessionstore/SessionFile.jsm:67:12 SessionSaverInternal._writeState@resource://app/modules/sessionstore/SessionSaver.jsm:264:19 SessionSaverInternal._saveState@resource://app/modules/sessionstore/SessionSaver.jsm:227:12 SessionSaverInternal.run@resource://app/modules/sessionstore/SessionSaver.jsm:129:12 this.SessionSaver<.run@resource://app/modules/sessionstore/SessionSaver.jsm:72:12 ssi_uninit@resource:///modules/sessionstore/SessionStore.jsm:492:7 ssi_onQuitApplication@resource:///modules/sessionstore/SessionStore.jsm:1115:5 ssi_observe@resource:///modules/sessionstore/SessionStore.jsm:520:9