Ideal temperature and humidity for preventing problematic ESD conditions?
Nobody relies on temperature and humidity to prevent ESD.  Some people might use it to minimize ESD, but they don't rely on it to eliminate the problem.

If you want to eliminate ESD as a potential problem then there is no replacement for proper equipment and proper handling of parts.  Ground straps, conductive/dissapative tools, floor, and furniture.  All the usual stuff.

Temperature does not matter much, but humidity does.  Temperature matters only in that it can effect humidity.  Ideally, the whole factory would be under water.  Clearly that is not practical, as it prevents the soldering irons from heating up.  So anything less than "underwater" is a compromise of ESD safety.  

Ironically, it is becoming common to store sensitive electronics in low-humidity environments.  Some chip packages, like BGAs, are humidity sensitive.  When stored in a humid environment they absorb water from the air.  Before soldering, the parts have to be baked to get that water out.  After soldering water absorption from the air isn't a factor (for reasons I don't completely understand).