Не удается установить предварительный просмотр веб-приложений Unity

Я пытаюсь установить unity-webapps-preview, поэтому ранее я добавил PPA:

    $ sudo add-apt-repository ppa:webapps/preview
    $ sudo apt-get update

, но получаю следующую ошибку:

    $ sudo apt-get install unity-webapps-preview
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies.
    unity-webapps-preview : Depends: xul-ext-unity but it is not going to be installed
                            Depends: xul-ext-websites-integration but it is not going to > be installed
                            Depends: xul-ext-webaccounts but it is not going to be installed
    E: Unable to correct problems, you have held broken packages.

Я пытался вручную установить зависимости, и я получаю:

    $ sudo apt-get install xul-ext-unity xul-ext-websites-integration xul-ext-webaccounts
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies.
    xul-ext-webaccounts : Breaks: firefox (>= 14.+) but 15.0.1+build1-0ubuntu0.12.04.1 is to be installed
    E: Unable to correct problems, you have held broken packages.

Версия Firefox: 15.0.1

Версия Ubuntu: 12.04