Mozilla's Mark Côté has reminded us that the arewefastyet page, which the browser maker launched prior to the introduction of Firefox 4, is still alive and is now tracking the progress of Mozilla IonMonkey JIT.
Though not an official statement made by Mozilla, Côté says that Firefox 18 (with IonMonkey) is now 16 percent faster than Chrome 21 in Mozilla's own Kraken benchmark, while it remains behind Chrome in V8 and behind Chrome in Sunspider. Mozilla's own test page suggests that IonMonkey has improved from a time of 1991 ms with the first release of IonMonkey in Firefox in mid-August to currently 1933 ms. Chrome has improved from 2038 ms to 2019 ms in the same time frame.
Mozilla does not yet support Google's latest browser benchmark Octane in its comparison. In a recent evaluation on Mozilla Hacks, Alon Zakai argued that Octane has some flaws and that there may be, for example, too much emphasis on Mandreel, which Google uses in its Chrome Web Store but may not be so important on web pages overall.
Zakai noted that "Octane is an interesting new benchmark that, even with [some] problems […], does contain good ideas and is worth focusing on." Yet he points out that no single benchmark is perfect and we "should always be aware of the limitations of any single benchmark, especially when a single benchmark claims to represent the entire modern web."