feature-wise firefox dev tools are pretty close to chrome's. However, if you have a large number (thousands) of sources and try to set a breakpoint, performance slows to a crawl. This probably doesn't impact a lot of web developers, but when you develop a large complicated web app and want to debug unobfuscated/unminimized code, this is a big pain point.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1288493