Having just posted my latest project, Asciible, that relies heavily (well, entirely) on client-side regular expressions, I've been paying closer attention to efficiency concerns. I don't know if I'd really consider myself to be a Firefox "fan boy", but it is my browser of choice, and I have to admit that I was surprised to find that IE7 was kicking FF2's butt when it came to executing String.replace().
To give you a real world context for my tests, Asciible simply searches through a text input and replaces special characters with their ASCII equivalents. Even all the custom options rely solely on simple String.replace()'s. So in fairness, be aware that this limited test does not represent an overall analysis of the regular expression engine. With that said, in this limited context IE was taking about 10% of the time it was taking Firefox to complete. I was even able to achieve nearly the same efficiency in FF2 by using a simple for loop and a character hash table - yuck.
After noticing this, I went and downloaded Firefox 3, Beta 4 and ran my tests again. I was disappointed to find only marginal improvements.
This is good news, especially since FF3 is still in beta. There's no guarantee that there will be further improvements in the final release, but the optimizations thus far are already outstanding, and certainly enough to leave me with my fingers crossed.