An excerpt from a letter written by Rav Wolbe (Iggros U'Kesavim vol. II pg. 214):
"Tonight is Lag Ba'omer and from the beginning of the zman the high school boys have been working to prepare a bonfire - and they succeeded in building a very large structure. After Shachris this morning I walked over to Rabbi . . . to express my objection to such a huge bonfire since it may burn down the trees in the area and who knows if it might even start a major conflagration chas v'shalom.
"I told him that regarding lighting the menorah on Chanukkah we find that Chazal placed the onus of preventing a fire on the camel driver [who passes by the lit candles with a sack full of flax] and not on the one who lights the menorah, since he is performing a mitzvah. (Even so, if one lights a menorah directly under a bundle of flax thereby causing the flax to get burned, he is certainly going to be liable to pay for the damage he caused with his own hands). This exemption is true regarding lighting a menorah which is a mitzvah. However, with regard to a bonfire on Lag Ba'omer - how can anyone be absolved from responsibility for directly or even indirectly causing damage?
"In addition, I told him that this is irrational thinking: to light a bonfire even if it will burn down a number of houses chas v'shalom. It was exactly about such things that Rav Yisrael Salanter z"l exhorted people lest they get caught up in a "frummeh negiah," and in order to perform their own mitzvah they might cause another great damage. (And who says that lighting a bonfire on Lag Ba'omer is a mitzvah altogether). Regarding such mitzvos Chazal declared, "He who makes a blessing [on stolen bread], has blasphemed Hashem!" A mitzvah must be entirely pure; untainted by an aveirah in any way."
Lag Ba'omer is a time where many people make bonfires and hundreds of thousands of people make their way to Meron. There is plenty of traffic, the buses are crowded and at any given time the kever of Rebbi Shimon Bar Yochai is jam-packed with people. We must ensure that the mitzvah and zechus of davening with the biggest "minyan" of people in the world, at the kever of one of the greatest people to ever live, should remain just that - a mitzvah and a zechus. If because of our zeal we cause others aggravation, it is better to stay home. This is a rule that applies everyday and everywhere, not just on Lag Ba'omer in Eretz Yisrael!