Help on text search on a page

I'm on a page. I select a piece of text. I hit the search keyboard shortcut (CTRL+F). Why is the text I selected not in the search box?

All code IDEAs do this, yet here am I on Firefox looking for something on the page that I last searched last year because yes.

It is something so simple but so infuriating sometimes. Is there a way to tweak this?