Firefox for Android 143 (and other versions) can be obtained from the Mozilla archives.

When browsing the archive, keep in mind that Firefox for Android is codenamed "Fenix".

So the following link would be for the latest stable version of v143:

https://archive.mozilla.org/pub/fenix/releases/143.0.4/android/

You then need to select the architecture of your Android device (or use " fenix-143.0.4-android" that supports all available ones, but is a larger download and install) to download and install the APK.

@goncalor: Thank you for asking this question and answering it. It helps to be able to download the APK from a reliable source. @Firefox developers: Android 7 is not such an old OS. Moreover, not everyone has so much wealth that they could keep buying new phones. Either Android should have an option to upgrade the OS to the latest version even on older phones or Firefox could continue offering the APK via Google Playstore for older versions. Please don't leave users in the lurch just because Android has been so poorly designed that it cannot be upgraded to the latest version. There is no really good alternative browser.

Hi goncalor,

Thank you so much for coming back to this thread to share the solution to your issue. We really appreciate it!

I just marked your answer as the solution to help others who might run into the same problem.