In most areas (though certainly not all), the difference was generally ironed out well before the advent of WebExtensions—what’s come since then has been as often surpassing as catching up.
Chrome was definitely a wake-up call, and the Firefox 4 cycle in 2010 achieved a lot. I recall doing audio stuff with the new Audio Data API (sadly since discontinued in favour of the much-more-complex-generally-for-no-good-reason Web Audio API) with a sine waves stress test (adding random sine waves together until underrun occurs) in Nightly, and watching the number it could cope with increase, week after week, due to JIT engine improvements. It went from handling a dozen to handling a couple of hundred over the course of two or three months.
Chrome was definitely a wake-up call, and the Firefox 4 cycle in 2010 achieved a lot. I recall doing audio stuff with the new Audio Data API (sadly since discontinued in favour of the much-more-complex-generally-for-no-good-reason Web Audio API) with a sine waves stress test (adding random sine waves together until underrun occurs) in Nightly, and watching the number it could cope with increase, week after week, due to JIT engine improvements. It went from handling a dozen to handling a couple of hundred over the course of two or three months.