Problem: I want to find other occurrences of a certain phrase on a webpage. I select the text, hit Ctrl+F, and to my disappointment, it still contains my last search (perhaps even from another webpage).
Proposal: Chromium-based browsers take the current text selection on the webpage and insert it into the search string when hitting Ctrl+F. I feel like implementing the same functionality in Firefox wouldn't be complicated, and I believe that most power users would find this rather useful.