Hey there 
I love the Master Password feature. I do have an issue with it, though.
The prompt that appears every now and then for me to input my MP does not clearly inform me that it comes from my open browser. It is a bit confusing at times.
Would there be a way to make it more obvious why the prompt pops up and which open window it originates from? It only says “Input password” right now.
I fully agree with this. I expect the popup to open at some point since I know that I restarted my browser recently. However, for a normal user this might not be clear at all. This gets even better if they had been told “Do not just randomly input your password everywhere” 
I wish there was a quick sign-out button on the Toolbar - for when a different user borrows the browser.
I use Firefox on several shared computers at work. Casual guests have no idea how to log out to get past the Master password, and I get people accidentally saving their passwords into my account - and worse logging into my accounts, because the Master time-out doesn’t work.
Would be nice with option to auto-remove bookmarks as well. Reverting to a sort-of default account for casual users.