There was a good long comprehensive Sacramento Bee article several years ago specifically about Western wildfires and firefighter arson, which unfortunately I can't find at the moment.
There's this from a South Carolina site in 1996:
FIREFIGHTER ARSON: LOCAL ALARM
...But when the headlines scream “Fireman Arrested for Arson,” it gets downright personal. The South Carolina Forestry Commission began looking closely at this phenomenon in 1993. By the end of the year, the tally of confirmed arrests was truly alarming—at least 33 fire department volunteers had been charged with arson. In 1994, 47 more were arrested. Forestry Commission and South Carolina Fire Service officials alike were astounded at the extent of the problem. “We knew it happened occasionally,” said Miles Knight, fire chief of the Forestry Commission, “but we were surprised by numbers like this.” Is this something relatively new, perhaps related to the popularity of “real-life emergency” television programming? Or is it a long standing problem, the magnitude of which had eluded the scrutiny of fire management professionals? Maybe these cases just didn’t stand out among the thousands of other arson incidents, and maybe sensitivity to the embarrassment of involved departments clouded our vision. We may never know for certain because most law enforcement records don’t routinely differentiate between arsonists who are firefighters and those who are not. Without comprehensive information, the natural tendency is to view each case as an isolated incident. This may be the case in other southern states as well. Most forestry agencies in the south acknowledge that firefighter arson does occur to some extent. The Alabama Forestry Commission says they investigated five or six cases in 1993; Arkansas has had five or six cases over the past few years; Kentucky has a “significant problem” with individuals setting fires so they can be hired to put them out; and Louisiana notes that wildland firefighters sometimes set fires to gain overtime pay. Some state law enforcement agencies in the south can tell you how many cases of firefighter arson they have prosecuted, but we found no single source of information in any of the states we polled. The situation is much the same with national fire agencies—nobody seems to know, and at least one major national fire agency denied having any knowledge of firefighter arson whatsoever.
Homeland Security has a 57 page report on the Problem of Firefighter Arson:
Firefighter Arson
Here's an article about the same problem in Australian wildfires:
Smoke screen
This is just from the first page of google hits.
Only a microscopic fraction of firefighters are arsonists, of course, and most arsonists aren't firefighters. It's shocking that the number of firefighter arsonists are so high, and it's interesting psychologically.
I have no evidence any of the current Southern California arsons are firefighter arsons, but nobody has any evidence it's Muslim terrorists. The ratio of the number of previous wildfires in the US known to be caused by firefighters to the number known to be caused by Muslim terrorists, is, however, infinity at the moment.
“There was a case in Boston several years ago where firefighters were accused of arson, and I believe convicted. It was in the Readville section of town. I did a quick Google search without finding an affirmation of this item.”
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Oh, I’m sure “some” wildfires have been set by firefighters. That’s a far cry from “a surprisingly large number” which is what the OP said.