Verification is needed for this one but I think MicroB is installed on the 256MB NAND flash while FF is installed on eMMC so naturally MicroB is going to be faster.

By slow I refer to scrolling down a webpage where it would pause for half a second and then continue to render. This IMHO is horrible. It spoils the experience and I have never faced this with MicroB. I frankly dont care how long it takes to startup a browser but the browsing itself should be reasonable.

Like I said I will give it the benefit of doubt being in beta but sorry it's not as fast as MicroB.

Edit: and yes I have tweaked FF myself since 1.0 days.