mozilla-firefox package name and content mismatch
 

The mozilla-firefox esr packages actually contain non-esr versions. I guess they, e.g. firefox-52.0.2 and firefox-52.0.2esr, are essentially the same software but only with different version strings, so everything works ok except when installing xpi files that check the version, where the xpi files for non-esr versions must be installed even though the slackware package is named mozilla-firefox esr.

So, could the mozilla-firefox esr packages be renamed to non-esr to avoid potential confusion?

The esr and non-esr versions are different.

O really? Didn't know that.

Anyway, it's confusing that I had no problem when using a firefox 52.0.1esr compiled by myself. Only when using slackware's official firefox esr packages (also before, not only 52.0.2), the xpi language packs for esr from mozilla don't work. Using the xpi file for the corresponding non-esr version works, e.g. using the official language pack for firefox-52.0.2 on slackware's firefox-52.0.2esr works.

For the latest firefox (esr 52.1.2) package for slackware64-14.2, still the xpi language pack for non-esr must be used. If the xpi language pack for 52.1.2esr is installed, the user interface stays in en_US.