Firefox 18.104.22.168 introduced a bug in the "Begin finding when you begin typing" feature.
This is a sporadic bug -- it occurs occasionally when you are filling out a form.
Instead of filling in a text box, it will think that you are intending to find something on the page. It should be aware that the cursor is currently in a form and not use the "Begin finding when you begin typing" code until you no longer have a form field in focus.
I have not found a reproduceable process yet to examine this error.