Firefox no longer gives automatic prompts when I type in first few letters of a search - even though my browsing history is being recorded, and "Remember search and form history" is ticked in the Privacy preferences: what's gone wrong?

Firefox has recently stopped giving (personalised) automatic prompts when I type in the first few letters of a search. What does come up is a long list of possible search subjects - but these are very general and don't reflect my own personal browsing history. My browsing history IS being recorded, however, and the "Remember search and form history" box is ticked in the Privacy preferences. I'm not using the "Private Browsing" option. There are two MacBooks in the house, sharing one wireless router. This problem has cropped up on both laptops - mine and my wife's - at around the same time.