When Brendan Eich was still there they seemed to have a clear purpose and identity. A lot of interesting stuff was happening: the birth of what would become Rust, PDF.js which is now ubiquitous back when downloading PDFs was the norm, Firefox OS, which somehow found success after they abandoned it, asm.js, the predecessor of web assembly which killed the alternative that Google was championing called NaCl (very few remember it now).
Then they just seemed to lose their way I can't tell how