OK, I opened /home/user/.mozilla/microb/cookies.txt and selectively deleted the cookies I didn't want. And everything worked properly -- like Amazon, which I didn't want to continue recognizing me, no longer recognized me; and The New York Times, which I wanted to recognize me, did. And I don't know if it made my life more private or my browser faster, but I got rid of a slew of other cookies and didn't break anything in the process.