I found it! I found a source for lighting fires on Lag B’Omer that goes back earlier than a few decades.
Rabbi Ovadiah Bartenura travelled to Israel and wrote letters to his brother back in Spain. These are published as Darkei Tzion, and are available online at HebrewBooks.org. If you look on page 17b (which is 33 of the scanned version) he says the following (this was written in 5149 – 1389):

On the 28th of Iyar on the day of his death (at this point the editor has ‘fixed things up and added: It seems that this is a scribal error, and should say 18th of Iyar which is the day that the G-dly Tanna Rabbi Shimon Bar Yochai died, and is his yoma hilula) people gather from all around and light great torches, apart from the one which they use to light the eternal light.

So we have a historical basis for the custom of lighting bonfires on Lag B’Omer. I still think this custom has pagan origins, but at least it wasn’t just made up in the past few weeks. The Bartenura doesn’t give any explanation or reason for this custom, and neither does Rabbi KiTov, whose book gave me the clue to find this.

Does anyone else know of any sources that discuss this custom? I know of one Jerusalem Shul Rav who stood up on Shabbat and told his congregation that it is forbidden to go to the bonfires, because apart from the (lack of) safety issue, the pyres are made from wood that has been stolen by the kids from all sorts of places, and it is forbidden to get benefit from stolen things.

OK, I suppose that is probably enough about bonfires, and I should go back to making fun of the government now (and especially the defense minister, who disagrees with the Wingrad commission’s findings, and thinks that he did an excellent job as defense minister – certainly better than all those war mongers with ‘experience’ would have done. At least he is honest enough to say that it would be a bad idea to have elections now because the wrong kind of people would get into government (i.e. anyone who isn’t him). The article is here – if you feel like a laugh. It is on JPost