Debian 9에 최신 버전의 Firefox를 설치하는 방법은 무엇입니까?

저는 Debian 9를 실행하고 있으며 해당 배포판과 함께 제공되는 매우 오래된 ESR 버전의 Firefox보다는 항상 최신 Firefox 버전을 사용하고 싶습니다.

그래서 /etc/apt/sources.list파일을 다음과 같이 변경했습니다.

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

이것은 한동안 잘 작동했습니다. 그런데 지금 메시지를 받았습니다.

The following packages have been kept back: firefox

"적성 업데이트 파이어폭스"를 실행할 때마다.

"aptitude dist-upgrade" 명령을 실행하면 다음과 같은 결과가 나타납니다.

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/?]

이 문제를 해결하는 데 도움이 되는 제안을 보내주셔서 진심으로 감사드립니다.

//편집: "aptpolicy"의 출력은 다음과 같습니다.

 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