There are Add-ons Manager Dialog Returns and Classicish Add-on Manager add-ons. Both open Add-ons Manager in separate window (Classicish can be set to open in tab) and it can be closed with Esc. Add-ons Manager Dialog Returns arguably has several advantages in that it doesn't need restart, doesn't change the appearance of Add-ons Manager from the current one (although that could very well be a bonus for Classicish, depending on personal preference) and it introduces a number of useful keyboard shortcuts (see AMO page for more info).
But you could also partially (in that it requires using a keyboard) get desired behavior without installing any add-on. Just press Ctrl+L or Alt+D when in Add-ons Manager and Open Location dialog will pop up, so you can just type address, being offered suggestions same as in Location Bar, and press Enter or click Open. It will open new address in current tab, replacing Add-ons Manager, by default. You can also use Ctrl+K or Ctrl+J to open your default search engine - first in the Search Bar list, not currently selected. There is also Alt+Home for Home Page.
Take your pick.
EDIT: Hide Navigation Bar add-on resolves it completely. It toggles Navigation Bar with F2 (changeable) by default, so all you have to do is open Add-ons Manager and make it visible.