The author admitted the stronger-than-needed title in the first comment, but I'd like to make it clear when talking about practical personal security:

There are ~100,000 people in the world that would think to run a GPU password cracker on your Firefox master password hash, if they had access and wanted to snoop.

There are ~1,000,000,000 people that would think to open your browser and go to a website you use to try to gain access to it.

In some sense, although it's hard to quantify security, using a master password is 10,000x more effective than not.