With Mac OS X 10.12 you can use the current release and it isn't recommended to install older version and specially 32-bit Firefox on Mac using a version that supports a universal binary.

I don't know when they started offering only 64-bit and stopped with the universal builds. What reasons do you have for going back to 32-bit Firefox ?

Firefox 53.0 (released way back on April 19, 2017) dropped support for 32-bit macOS as it was 64-bit only from that point.