EDIT: Forgot to mention that issue appears even after creating a brand new profile.

This is version 115 and translations are available since ~118.

From

The translation feature was rolled out gradually and has been available for all users since Firefox version 118. A new Translate selection feature has been added starting in Firefox version 128, that enables you to highlight and translate selected text.

Firefox 115 ESR is a old ESR channel based on the old Firefox 115.0 Release and the updates are only for security and stability fixes and not features. It turns out the old Fx 115 ESR may be getting updates til Fx 115.21.0esr in March now for Windows 7 - 8.1 and macOS 10.12 - 10.14 users.

Thank you, just confirmed that another ESR version also lacks the translate functionality available on RR. Thought that the ESR and RR version numbers could not be "compared", but seems I was wrong...

Would installing Firefox translations addon ( https://addons.mozilla.org/el/firefox/addon/firefox-translations ) on ESR do the trick for me? Or could it produce any due to my Firefox sync'ing between ESR and non-ESR versions?

Firefox 115 ESR is a old ESR channel based on the old Firefox 115.0 Release and the updates are only for security and stability fixes and not features. It turns out the old Fx 115 ESR may be getting updates til Fx 115.21.0esr in March now for Windows 7 - 8.1 and macOS 10.12 - 10.14 users.

Thx James. Out of curiosity, what happens after 115.21.0, that is which RR version will be elected as a new ESR? Any links on how the new ESR election takes place?

Thank you, just confirmed that another ESR version also lacks the translate functionality available on RR. Thought that the ESR and RR version numbers could not be "compared", but seems I was wrong... Would installing Firefox translations addon ( https://addons.mozilla.org/el/firefox/addon/firefox-translations ) on ESR do the trick for me? Or could it produce any due to my Firefox sync'ing between ESR and non-ESR versions?

Firefox 115.0 ESR is based on the Firefox 115.0 Release with the updates being only for allowed security and stability fixes and not features. The more current Fx 128.0 ESR is based on the Firefox 128.0 Release but with allowed security and stability fixes since.

From https://support.mozilla.org/en-US/kb/how-add-translate-feature-firefox

Changes to the Firefox translations add-on With the introduction of the built-in translation feature, the Firefox Translations add-on will no longer receive updates or maintenance. While the add-on will remain functional for a period, we encourage users to transition to the built-in translation feature for the most up-to-date and secure experience.

Thx James. Out of curiosity, what happens after 115.21.0, that is which RR version will be elected as a new ESR? Any links on how the new ESR election takes place?

Originally Fx 115.15.0esr was going to be EOL as of Oct 1st as the current versions was going to be Firefox 131.0 Release and Firefox 128.3.0esr.

When Fx 115.21.0esr comes out if it gets that far as decision will be made in early 2025, the versions will be Firefox 136, Firefox 128.8.0esr and Firefox 115.21esr.

The next major version the ESR will be based on in future will not be until the Firefox 140.0 Release.