>I like having an unlimited number of "from addresses"

>The per folder preferences make it trivial to deal with both personal and work email from my runbox account.

How to access these functions?

 
At the top of your folder list, you should see a link marked "Preferences". Click on it, to change the "From Address".

Each folder has its own set of preferences. Simply click any folder and then click the "Preferences" button.