Debian 9に最新バージョンのFirefoxをインストールするには？

私はDebian 9を実行しており、そのディストリビューションに付属している非常に古いESRバージョンのFirefoxではなく、常に最新のFirefoxバージョンを使用したいと思います。

そのため、/etc/apt/sources.listファイルを次のように変更しました。

deb http://ftp.de.debian.org/debian/ stretch main
deb-src http://ftp.de.debian.org/debian/ stretch main

deb http://security.debian.org/ stretch/updates main contrib
deb-src http://security.debian.org/ stretch/updates main contrib

# jessie-updates, previously known as 'volatile'
deb http://ftp.de.debian.org/debian/ stretch-updates main contrib
deb-src http://ftp.de.debian.org/debian/ stretch-updates main contrib

deb http://ftp.us.debian.org/debian/ sid main
deb-src http://ftp.us.debian.org/debian/ sid main

これはしばらくうまくいきました。ところで今メッセージを受けました。

The following packages have been kept back: firefox

「適性更新 Firefox」を実行するたびに。

"aptitude dist-upgrade"コマンドを実行すると、次の結果が表示されます。

The following packages will be DOWNGRADED:
libc-bin libc-dev-bin libc-l10n libc6 libc6-dev locales 
The following packages will be upgraded:
firefox{b} 
1 packages upgraded, 0 newly installed, 6 downgraded, 0 to remove and 0 not upgraded.

Need to get 51.2 MB of archives. After unpacking 12.0 MB will be freed.
The following packages have unmet dependencies:
firefox : Depends: libc6 (>= 2.27) but 2.24-11+deb9u3 is to be installed
       Depends: zlib1g (>= 1:1.2.11.dfsg) but 1:1.2.8.dfsg-5 is installed
The following actions will resolve these dependencies:

 Keep the following packages at their current version:
 1)     firefox [59.0.2-1 (now)]                           
 2)     libc-bin [2.27-3 (now, unstable)]                  
 3)     libc-dev-bin [2.27-3 (now, unstable)]              
 4)     libc-l10n [2.27-3 (now, unstable)]                 
 5)     libc6 [2.27-3 (now, unstable)]                     
 6)     libc6-dev [2.27-3 (now, unstable)]                 
 7)     locales [2.27-3 (now, unstable)]                   
Accept this solution? [Y/n/q/?]

この問題を解決するのに役立つ提案を送っていただきありがとうございます。

//編集： "aptpolicy"の出力は次のとおりです。

 100 /var/lib/dpkg/status
 release a=now
 500 https://riot.im/packages/debian stretch/main amd64 Packages
 release o=riot.im,a=testing,n=stretch,c=main,b=amd64
 origin riot.im
 2 http://ftp.us.debian.org/debian sid/main amd64 Packages
 release o=Debian,a=unstable,n=sid,l=Debian,c=main,b=amd64
 origin ftp.us.debian.org
 500 http://ftp.de.debian.org/debian stretch-updates/main amd64 Packages
 release o=Debian,a=stable-updates,n=stretch updates,l=Debian,c=main,b=amd64
 origin ftp.de.debian.org
 1000 http://security.debian.org stretch/updates/contrib amd64 Packages
 release v=9,o=Debian,a=stable,n=stretch,l=Debian-Security,c=contrib,b=amd64
 origin security.debian.org
 1000 http://security.debian.org stretch/updates/main amd64 Packages
 release v=9,o=Debian,a=stable,n=stretch,l=Debian-Security,c=main,b=amd64
 origin security.debian.org
 1000 http://ftp.de.debian.org/debian stretch/main amd64 Packages
 release v=9.4,o=Debian,a=stable,n=stretch,l=Debian,c=main,b=amd64
 origin ftp.de.debian.org
 Pinned packages:
 fontconfig-config -> 2.13.0-5 with priority 1001
 libfontconfig1 -> 2.13.0-5 with priority 1001
 libnss3 -> 2:3.37.1-1 with priority 1001
 firefox -> 60.0.1-5 with priority 1001