Always disconnect the hot wire first. If you disconnect the neutral first, then it will become energized. You should then reconnect the wires in reverse order (hot last).
Keep in mind that the Code now requires that all commercial fluorescent lighting fixtures have a switch or other approved disconnect installed on the fixture to de-energize it when servicing. You should add this switch when replacing the ballast.

Sounds like the switch may be going bad.
While fire is a remote possibility, the more likely problem will be that the switch will fail sometime in the future.

I’m replacing my outside post lamp that has a built-in photocell. The new light doesn’t. What wires of my photocell do I connect to the new light? Do I use the bare copper wire on the light?

The bare copper wire is for grounding only - not for current carrying.
A photocell would have a hot wire (black), a neutral (white), and a load wire (usu. red). The load wire would connect to the hot wire of the lights you would want to control via the photocell.

The best solution is to replace the fixtures with ones that have accept screw-in incandescent bulbs. You could open the fixture, change the socket, etc. but keep in mind that this will void the UL listing on the fixture.

My 12 volt DC pendant light and a replacement one is over $81.00. So I found another light with the same transformer (I thought). I put the new transformer on the old fixture and it blew the bulb. Then I noticed the new transformer was 12 volts AC. The original fixture looks fine. Do you think I can get a 12VDC transformer and it should still work?

First of all, unless you purchase the replacement parts from the same manufacturer of the original fixture, you won't be able to guarantee that it will work properly and avoid a potential problem such as fire, etc. Secondly, transformers can only convert AC voltage of one potential to AC voltage of a different potential (i.e. 120V to 24V). Transformers can’t convert AC to DC – this requires the use of a power supply.