Don't discharge a fire extinguisher in flight unless you want to create an IMC condition. Spraying white powder everywhere will cling to the screen and you will be blind! Now try to auto!
There is a reason (at least it was explained to me in Oz) for the fire extinguisher to be on the pax side, so the pilot can't do it alone. The pilot tells the pax to open the door before firing it off. It's also located a long way forward, watch the C of G move when the pax reaches for it. Also watch the cyclic when the pax does reach forward. We tried this one day and the pax had trouble reaching the extinguisher without hitting the cyclic. As the pax and C of G moved I needed more left rear cyclic. The pax was pushing the cyclic forward.
We were just practicing a drill and there was no fire.
Action on electrical fire? Land ASAP, then fight the fire (or run).
What can cause it? As was stated above, only after market stuff that's fitted will burn. We had a UHF CB radio fitted for talking to trucks/marine and it shorted out the panel. That's one cause.