Doesn’t do that for me. Have you got some oddball extension installed or something?

Found that this is a Firefox setting, maybe it's not (no longer) defaulted to on.

"Search for text when you start typing"

I have to say, I do like this setting enabled, but can see how it conflicts with the page. And let's be fair, how much time and I saving over having to press Ctrl+F when I want to search a page?