Opposition supporters celebrated outside the courthouse by lighting firecrackers.

People celebrate by letting off firecrackers. Would you say lighting firecrackers?

You can light a cigarette or you can light a fire.
Though I've never done it myself, Rotter, 'letting off' a firecracker, or any of the other fireworks, cannot be done without first 'lighting' them--meaning touch a match (or something equivalent) to them

Do you know the song, The Battle of New Orleans by Jimmy Driftwood, popular in the 1950s? Here's part of the lyrics:

Well, we fired our cannon til the barrel melted down,
so we grabbed an alligator and we fought another round.
We filled his head with cannon balls and powdered his behind,
and when they tetched the powder off, the gator lost his mind.

'Tetched (touched) the powder off' = 'lighting'

'Lost his mind' = 'letting off'
I would say "setting off firecrackers".