So all or most fire benders can bend lightning now? Lame.
Not necessarily. We saw about 5 people(counting Mako) working in a factory to power it. The pay was good, so it must not have been too popular a job, meaning lightning bending isn't as popular as we think. Zolt is a crime lord, I'd expect him to know more than the average bender. Also, we must take into consideration that the Royal Family were the only ones we saw using lightning. This is likely because they were the only ones who had the actual training and knowledge. I'm sure once the fire nation came under Zuko's rule, the technique was spread around much more. So, I believe it's fair to say all it takes is knowledge and a "lack of inner turmoil."
Fair enough. But I prefer lightning bending to be a technique that only a select few fire benders are able to perform.