I’ve been running the very latest Firefox for a while now, and I kept getting problems with source maps not working correctly. I finally decided to dig into this, and found out that it was :sourcemap-timestamps that was creating the problem
This is an option that you can set in the ClojureScript compiler options, it adds ?rel=<timestamp> to the source map links to make sure they don’t go stale, but it seems to trip firefox up.
So long story short, if you’re having trouble with source maps, try turning this option off.