There are some differences between microb and firefox mobile in zooming. Microb automatically zooms out a web page if it's too big, but only as far as readability allows. It won't let the text get too small. Firefox mobile does not have this restriction, but rather always zooms all out giving an overview of the page to zoom in for closer inspection. Which is better is a matter of opinion.
Firefox has the double tap zoom like microb, but without the smooth animation. I like that animation. Firefox does not have the screw-in zoom, which I almost never use anyway. Both use the volume keys to adjust zoom.
Text looks more blurry in firefox when zoomed out. Still readable, but blurry. Microb keeps the text sharp.
A small, but noticeable difference in integration is the location of the system menu button (typically in the top left corner). In firefox it's in the address bar, at the top of each page. If you're reading a long page and want to check an email, you have to (well, no, as there is a key combo, but not everyone knows it) scroll to the top of the page losing your place in the page to switch applications. I'd rather have it in the left side bar (tab bar) for quick access.
