I bumped a group of keys on my keyboard, which evidently toggled on a “feature,” by which every time I hit a / or a ‘ (slash or apostrophe), a “Quicksearch” box opens at the bottom of the window.
Even after consulting Firefox’s “Help” forums, the only way to stop it was to do a System Restore. It hasn’t come back—yet.
It still happens when I’m reading a page, but not when I am typing in a data/text field, like right now.
That is your “Find” feature cranking up with the expectation you are searching for text on the page. I was so used to hitting “Ctrl-F” that it was annoting at first. Now if I accidentally hit a key I just hit escape to get out of it. Not sure if there is a configuration feature to manage this but I would imagine there is. If it really bugs ya, try binging the config features of “about:config”