Recently, I had the latest Firefox 40.0 version installed on my computer.
A strange problem had then popped up on my HP PC: When I started entering letters in the "Search or enter address" window of Firefox, the first few letters appeared extremely faint and thus unreadable, though they reappeared when I kept typing the search letters.
I thought of replacing Firefox 40 by its previous version 39 as a possible remedy. The remedy worked!
I am entering this post only for the awareness of other Firefox users as to what worked for me.