Now that wildfires are such a huge issue everywhere it's a wonder that the word isn't in everyday use.
Such paths, or glades, don't always prevent wildfires, but they sure make the firefighter's job a lot easier.
What are they called?
I've dug up a picture of one; it's from a German page, and the German word for it is "Schneise."
Best Answer
Although firebreak is the most common term, you could consider using fireroad, fire line or fuel break as suggested by the Wikipedia article.
Fuel break is defined by Natural Resources Conservation Service as:
A picture of fuel break:
A picture of fire line: