I am in Firefox or Chrome, I press Ctrl+F and sometimes the browser search box appears. But some other times the FORUM SEARCH functionality kicks in. See bellow picture:
I really prefer the browser one. I cannot understand what is different, it seems to depend on the particular forum page.
On my end Ctrl+F brings up browser page search. Note that browser page search does NOT search the forum. It just searches what is currently displayed on screen which is NOT full content (due to the fact that posts are dynamically loaded as you scroll)