Horsetail Falls lights up like a fire falls during only one time of the year - roughly the last 2-3 weeks of February. It's only at this time when the setting sun hits the west face of El Capitan at the proper angle. However, a number of conditions all have to be present:
1. Time of year (as already mentioned)
2. Sufficient snow runoff to cause Horsetail falls to fall
3. Clear enough weather at sunset to allow direct sunlight to strike the falls
Speaking of Steve Kossack, I attended his February 2006 Yosemite winter workshop and we were fortunate enough to photograph Horsetail falls from one of the classic locations, Sentinel Beach:http://www.pbase.com/pauls/yosemite_2006
BTW, weather and snow conditions in Yosemite can vary widely during the winter. When I was there in February 2006, conditions were almost Spring-like with very little snow on the Valley floor. But 2 weeks later, friends encountered a full-on snow storm.
Chains are required to be carried and used by law in and around Yosemite NP under the following snow conditions:http://www.nps.gov/yose/planyourvisit/chains.htm
It appears that even with snow tires, chains are required to be carried.
Finally - live webcams of Yosemite:http://www.yosemite.org/vryos/index.htm