Very helpful. Has a flaw though. Rated 4 out of 5 stars

Most useful addon for me!! Really convenient to use and serves the purpose completely.

However there is one thing, I would like to point out. This utilizes Mozilla's Saved Password utility for saving the passwords and therefore, has proper checks in place but one. The button, allowing the user to copy the password, is a weakness that can be exploited. I say this because, the same can then be pasted in any field thereby revealing it. And the username is always visible in Saved Password utility anyhow.
All this, while the master password is enabled.

The addon already has the functionality to copy the credentials in context menu and so the copy button could be done away with.

This review is for a previous version of the add-on (2.6.1-signed). 

it is fixed now...

...Thanks for the feedback. I know that once you get into the password manager window there are no additional checks, and I thought long and hard about whether they should be introduced - like they are when the user clicks on the [Show Passwords] button. IMO asking for the Master Password once is sufficient; I want to avoid for the user having to do any additional clicks or actions.

So in 2.7 I have added a "Lock" icon to the password manager - if you plan on leaving your browser open unattended, simply click it to snap the lock shut before you close it.

The password manager will then ask for the master password next time you use QuickPasswords or do any password related operation. Here is the related feature request pagehttps://www.mozdev.org/bugs/show_bug.cgi?id=25319