ошибка установки Firefox

Я получаю ошибку при установке любой стабильной версии пакета Firefox:

Пошаговая установка:

1. сначала проверка, установлен ли Firefox или нет:

   $ firefox
   The program 'firefox' is currently not installed. You can install it by typing:
   sudo apt-get install firefox
   

2. Firefox установки:

   Reading package lists... Done
   Building dependency tree
   Reading state information... Done
   The following extra packages will be installed:
     libcanberra0 libogg0 libstartup-notification0 libtdb1 libvorbis0a
     libvorbisfile3 libxcb-util0 xul-ext-ubufox
   Suggested packages:
     fonts-lyx libcanberra-gtk0 libcanberra-pulse
   The following NEW packages will be installed:
     firefox libcanberra0 libogg0 libstartup-notification0 libtdb1 libvorbis0a
     libvorbisfile3 libxcb-util0 xul-ext-ubufox
   0 upgraded, 9 newly installed, 0 to remove and 29 not upgraded.
   Need to get 45.3 MB/45.6 MB of archives.
   After this operation, 113 MB of additional disk space will be used.
   Do you want to continue? [Y/n]y
   Get:1 http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu/ trusty/main firefox amd64 50.0+build2-0ubuntu0.14.04.1 [45.3 MB]
   Fetched 45.3 MB in 9s (4,880 kB/s)
   Selecting previously unselected package libtdb1:amd64.
   dpkg: warning: files list file for package 'firefox-mozilla-build' missing; assuming package has no files currently installed
   (Reading database ... 94637 files and directories currently installed.)
   Preparing to unpack .../libtdb1_1.3.8-0ubuntu0.14.04.1_amd64.deb ...
   Unpacking libtdb1:amd64 (1.3.8-0ubuntu0.14.04.1) ...
   Selecting previously unselected package libogg0:amd64.
   Preparing to unpack .../libogg0_1.3.1-1ubuntu1_amd64.deb ...
   Unpacking libogg0:amd64 (1.3.1-1ubuntu1) ...
   Selecting previously unselected package libvorbis0a:amd64.
   Preparing to unpack .../libvorbis0a_1.3.2-1.3ubuntu1_amd64.deb ...
   Unpacking libvorbis0a:amd64 (1.3.2-1.3ubuntu1) ...
   Selecting previously unselected package libvorbisfile3:amd64.
   Preparing to unpack .../libvorbisfile3_1.3.2-1.3ubuntu1_amd64.deb ...
   Unpacking libvorbisfile3:amd64 (1.3.2-1.3ubuntu1) ...
   Selecting previously unselected package libcanberra0:amd64.
   Preparing to unpack .../libcanberra0_0.30-0ubuntu3_amd64.deb ...
   Unpacking libcanberra0:amd64 (0.30-0ubuntu3) ...
   Selecting previously unselected package libxcb-util0:amd64.
   Preparing to unpack .../libxcb-util0_0.3.8-2ubuntu1_amd64.deb ...
   Unpacking libxcb-util0:amd64 (0.3.8-2ubuntu1) ...
   Selecting previously unselected package libstartup-notification0:amd64.
   Preparing to unpack .../libstartup-notification0_0.12-3ubuntu1_amd64.deb ...
   Unpacking libstartup-notification0:amd64 (0.12-3ubuntu1) ...
   Selecting previously unselected package firefox.
   Preparing to unpack .../firefox_50.0+build2-0ubuntu0.14.04.1_amd64.deb ...
   Unpacking firefox (50.0+build2-0ubuntu0.14.04.1) ...
   Selecting previously unselected package xul-ext-ubufox.
   Preparing to unpack .../xul-ext-ubufox_3.2-0ubuntu0.14.04.1_all.deb ...
   Unpacking xul-ext-ubufox (3.2-0ubuntu0.14.04.1) ...
   Processing triggers for mime-support (3.54ubuntu1.1) ...
   Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
   Setting up libtdb1:amd64 (1.3.8-0ubuntu0.14.04.1) ...
   Setting up libogg0:amd64 (1.3.1-1ubuntu1) ...
   Setting up libvorbis0a:amd64 (1.3.2-1.3ubuntu1) ...
   Setting up libvorbisfile3:amd64 (1.3.2-1.3ubuntu1) ...
   Setting up libcanberra0:amd64 (0.30-0ubuntu3) ...
   Setting up libxcb-util0:amd64 (0.3.8-2ubuntu1) ...
   Setting up libstartup-notification0:amd64 (0.12-3ubuntu1) ...
   Setting up firefox (50.0+build2-0ubuntu0.14.04.1) ...
   update-alternatives: error: alternative path /usr/bin/firefox doesn't exist
   dpkg: error processing package firefox (--configure):
    subprocess installed post-installation script returned error exit status 2
   Setting up xul-ext-ubufox (3.2-0ubuntu0.14.04.1) ...
   Processing triggers for libc-bin (2.19-0ubuntu6.9) ...
   Errors were encountered while processing:
    firefox
   E: Sub-process /usr/bin/dpkg returned an error code (1)
   

Таким образом, какова эта ошибка точно. Я искал об этом, но не способный решить его.

apt-cache policy firefox:

firefox:
  Installed: 50.0+build2-0ubuntu0.14.04.1
  Candidate: 50.0+build2-0ubuntu0.14.04.1
  Version table:
 *** 50.0+build2-0ubuntu0.14.04.1 0
        500 http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status
     50.0~b11+build1-0ubuntu0.14.04.1 0
        500 http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu/ trusty/main amd64 Packages
     49.0.2+build2-0ubuntu0.14.04.1 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     28.0+build2-0ubuntu2 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

Как я могу переключиться назад на ответвление стабильной версии Firefox?