What do you call a glade or path created artificially in order to to prevent wildfires?

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 strip or block of land on which the vegetation, debris and detrius have been reduced and/or modified to control or diminish the risk of the spread of fire crossing the strip or block of land.

A picture of fuel break:

A picture of fire line:

Such swaths are called "firebreaks":

firebreak (n.) - a strip of plowed or cleared land made to check the spread of a prairie or forest fire.

http://dictionary.reference.com/browse/firebreak?s=t

A straight avenue cut through a forest is often called a "ride". I imagine this is because they were once used for people on horseback to ride through the forest. If the purpose is to limit the spread of wildfire, then in Australia we would call it a "firebreak".