What they offer these extensions, they are about equal in their use and profit, is to have quick access to the features of the bar WordPress.com shared but not on WordPress.com, from the toolbar of your browser, and visit the site you visit .
Just install it and have the icon, and the first thing you have to do is enter your credentials WordPress.com.
From there the magic begins then, as with the WordPress bar, but from any website , you can follow a WordPress.com site if it is or has JetPack installed with integration, to " reblog "the same , and get out without serving notices of comments on your sites.
blogging site is wonderful, because you have a " Post it "but the style WordPress.com .
In short, I would say that extensions must for any wordpressero it claims to be. So you're taking on install, here are the links:
WordPress Extension for Chrome
Firefox Extension for WordPress
I do not use much these browsers but if I have installed Chrome , I use it when I'm traveling, to have everything in sync.