It was never included, because that certificate is a Comodo intermediate. It is the websites responsibility to provide the full certificate chain, and the error you see results because the site fails to do this (although, you didn't even provide a URL for me to check, but this is the most likely scenario).
This is explained in https://wiki.ubuntu.com/MozillaTeam/Bugs#A.22Secure_Connection_Failed.22_errors. In future, please read the bug reporting guidelines before reporting a bug. The link is provided on the bug submission form, along with a request to read it. ** Changed in: firefox (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to firefox in Ubuntu. https://bugs.launchpad.net/bugs/1012644 Title: Firefox 13.0 doesn't accept Comodo Certificates anymore Status in “firefox” package in Ubuntu: Invalid Bug description: Since update to 13.0 Firefox seems to have this Comodo cert removed (see screenshot) ProblemType: Bug DistroRelease: Ubuntu 12.04 Package: firefox 13.0+build1-0ubuntu0.12.04.1 ProcVersionSignature: Ubuntu 3.2.0-24.39-generic-pae 3.2.16 Uname: Linux 3.2.0-24-generic-pae i686 NonfreeKernelModules: fglrx AddonCompatCheckDisabled: False AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.24. AplayDevices: **** List of PLAYBACK Hardware Devices **** card 0: Intel [HDA Intel], device 0: ALC269VB Analog [ALC269VB Analog] Subdevices: 1/1 Subdevice #0: subdevice #0 ApportVersion: 2.0.1-0ubuntu8 Architecture: i386 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC0: mlopes 2722 F.... pulseaudio BuildID: 20120601173917 CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found. Card0.Amixer.info: Card hw:0 'Intel'/'HDA Intel at 0xff87c000 irq 44' Mixer name : 'Realtek ALC269VB' Components : 'HDA:10ec0269,102802da,00100100' Controls : 26 Simple ctrls : 14 Channel: release Date: Wed Jun 13 14:30:17 2012 EcryptfsInUse: Yes ForcedLayersAccel: False IfupdownConfig: auto lo iface lo inet loopback InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release i386 (20101007) IpRoute: default via 192.168.77.1 dev eth0 proto static 169.254.0.0/16 dev eth0 scope link metric 1000 192.168.76.0/23 dev eth0 proto kernel scope link src 192.168.77.216 metric 1 192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1 IwConfig: lo no wireless extensions. virbr0 no wireless extensions. eth0 no wireless extensions. ProcEnviron: PATH=(custom, no user) LANG=fr_FR.UTF-8 SHELL=/bin/bash Profiles: Profile0 (Default) - LastVersion=13.0/20120601173917 (In use) RelatedPackageVersions: adobe-flashplugin 11.2.202.236-0precise1 icedtea-6-plugin 1.2-2ubuntu1 rhythmbox-mozilla 2.96-0ubuntu4 totem-mozilla 3.0.1-0ubuntu21 RfKill: RunningIncompatibleAddons: False SourcePackage: firefox UpgradeStatus: Upgraded to precise on 2012-04-27 (47 days ago) dmi.bios.date: 11/03/2010 dmi.bios.vendor: Dell Inc. dmi.bios.version: A05 dmi.board.name: 0D441T dmi.board.vendor: Dell Inc. dmi.board.version: A03 dmi.chassis.type: 6 dmi.chassis.vendor: Dell Inc. dmi.modalias: dmi:bvnDellInc.:bvrA05:bd11/03/2010:svnDellInc.:pnOptiPlex980:pvr:rvnDellInc.:rn0D441T:rvrA03:cvnDellInc.:ct6:cvr: dmi.product.name: OptiPlex 980 dmi.sys.vendor: Dell Inc. ftp_proxy: ftp://proxy.ccfd.asso.fr:8080/ http_proxy: http://proxy.ccfd.asso.fr:8080/ To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1012644/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp