A great step Firefox could take in this space is improving the transparency and performance auditing of its own engine. Debugging rendering performance issues between browsers is still an absolute nightmare. If Firefox can provide a better developer experience than Chrome, it might increase the likelihood that web developers would use FF as their first-choice development platform for simplicity and utility of tools. If it doesn't, it may still decrease the odds that when faced with a nasty performance bug, developers will throw up their hands and say "Chrome does it right; I guess FF still just sucks for performance in this corner case."
FF has been making impressive strides in this space, but their toolchain still feels clunky and slow relative to Chrome Developer Tools.