Find bar (ctrl+f) neglects the input if it manages to find the word you are typing. As for an example : You want to find the word "troubleshooting" in a page. When you start to write the word, "as long as it is present on that page", firefox finds the word "tro" or "troub" and neglects what you type next. The length of this initial word depends on your typing speed i think (3-4 for me). And when you want to continue writing the word's remaing part, by selecting it with a mouse click or ctrl+f and right arrow, it keeps doing the neglection for each few (1-2) letter you type. It is not a big issue for 4-5 lettered words but after you repeat to type your words 50-60 times in an hour, it becomes annoying.