Origin of Serious Wildfires
There was an interesting paper on this for the United States a couple of years ago:

Balch et al (2017) Human-started wildfires expand the fire niche across
the United States, PNAS, doi: 10.1073/pnas.1617394114

They estimate that in the coterminous U.S. human-started events account "for 84% of all wildfires and 44% of total area burned".  There is a marked spatial variation, with lightning-started wildfires occurring mainly in the more sparsely populated western forests.  If you have access, the paper's worth a read, it has lots of interesting figures picking apart the details.



It's worth noting that there are a number of reasons for human-started wildfires, which are could be classed as accidental, deliberate or careless:


  Human-started wildfires were caused by a variety of sources, including the US Forest Service-designated categories of equipment use, smoking, campfire, railroad, arson, debris burning, children, fireworks, power line, structure, and miscellaneous fires.


They also note that their data contains this, perhaps not unsurprising, feature:


  There was also a notable mark of American culture on the distribution of wildfires, with the peak day of wildfires occurring on July 4th, concurrent with Independence Day fireworks displays (Fig. 2). Indeed, Americans start over twice as many wildfires on July 4th as any other summer day.