Most browsers v-sync the display to avoid tearing, so if a browser can only achieve 59 FPS, it might end up only rendering alternate frames and falling all the way to 30 FPS. (In other words since the time to render one frame could be *just above* the time it takes for the display to update, it is always still rendering when the display updates, so it has to wait for the next display update to show what was drawn.) So it may be that Firefox is only fractionally slower than Chrome, which seems likely.