1) Download version 46 of Firefox as required.

2) Install version 46

3) Open Firefox

4) Firefox then auto updates to version 52

5) Face smash desk and repeat

Why would any product ever require "version x" instead of "version x and above"?
This should not happen, updating should be encouraged.

If someone needs you to run a specific firefox version (because of some application relying on old behaviours) they should use an ESR release (FF 45 and FF 52 are available as ESR versions, they'll still get security fixes and won't automatically update to a newer release)

Just create a profile that has auto update disabled.

I'm more familiar with this than one ought to be.