Debian上的apt软件包管理器将软件包列出为可升级但未升级

此问题类似于Debian Linux上未更新的软件包，但就我而言，较新的软件包并非来自backports：

§ apt list --upgradable
Listing... Done
firefox-esr-l10n-en-gb/stable 60.2.0esr-1~deb9u2 all [upgradable from: 52.9.0esr-1~deb9u1]
N: There is 1 additional version. Please use the '-a' switch to see it

§ sudo apt upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

§ apt policy firefox-esr-l10n-en-gb
firefox-esr-l10n-en-gb:
  Installed: 52.9.0esr-1~deb9u1
  Candidate: 60.2.0esr-1~deb9u2
  Version table:
     60.2.0esr-1~deb9u2 500
        500 http://security.debian.org stretch/updates/main amd64 Packages
 *** 52.9.0esr-1~deb9u1 500
        500 http://ftp.fr.debian.org/debian stretch/main amd64 Packages
        500 http://security.debian.org stretch/updates/main amd64 Packages
        100 /var/lib/dpkg/status

§ apt policy firefox-esr
firefox-esr:
  Installed: 52.9.0esr-1~deb9u1
  Candidate: 52.9.0esr-1~deb9u1
  Version table:
     60.2.0esr-1~deb9u2 500
        500 http://security.debian.org stretch/updates/main amd64 Packages
 *** 52.9.0esr-1~deb9u1 30000
        500 http://ftp.fr.debian.org/debian stretch/main amd64 Packages
        100 /var/lib/dpkg/status

§ apt -s install firefox-esr=60.2.0esr-1~deb9u2
NOTE: This is only a simulation!
      apt needs root privileges for real execution.
      Keep also in mind that locking is deactivated,
      so don't depend on the relevance to the real current situation!
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  firefox-esr-l10n-en-gb
Suggested packages:
  fonts-stix | otf-stix
The following packages will be upgraded:
  firefox-esr firefox-esr-l10n-en-gb
2 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Inst firefox-esr-l10n-en-gb [52.9.0esr-1~deb9u1] (60.2.0esr-1~deb9u2 Debian-Security:9/stable [all]) []
Inst firefox-esr [52.9.0esr-1~deb9u1] (60.2.0esr-1~deb9u2 Debian-Security:9/stable [amd64])
Conf firefox-esr-l10n-en-gb (60.2.0esr-1~deb9u2 Debian-Security:9/stable [all])
Conf firefox-esr (60.2.0esr-1~deb9u2 Debian-Security:9/stable [amd64])

这是什么意思？为什么firefox-esr不升级？

更新资料

在斯蒂芬·基特（Stephen Kitt）回答后，我发现了一个文件/etc/apt/preferences.d/apt-listbugs，该文件是在今天早上运行升级时创建的，其内容如下：

Explanation: Pinned by apt-listbugs at 2018-09-11 08:11:30 +0200
Explanation:   #908396: firefox-esr: stopped working after upgrade from 59 to 60
Explanation:   #908449: (no subject)
Package: firefox-esr
Pin: version 52.9.0esr-1~deb9u1
Pin-Priority: 30000

这是什么意思？

您已经firefox-esr以很高的优先级固定到当前安装的版本：

 *** 52.9.0esr-1~deb9u1 30000

这样可以防止安装任何其他具有较低插针优先级的版本，包括仅具有优先级500的安全更新：

     60.2.0esr-1~deb9u2 500

引脚来自apt-listbugs：它决定（或被告知-我认为它总是会问，但可能有些配置没有这样做），应该暂停对Firefox 60的升级，直到修复了908396和908449错误为止。这些bug与SSE2指令的新要求有关i386; 因为您正在运行amd64，所以他们不关心您，因此升级应该是安全的。

您需要删除引脚优先级才能继续升级。删除/etc/apt/preferences.d/apt-listbugs，如果apt-listbugs再次询问您，请告诉您确实要升级。