Those keys aren't being typed, nothing with CTRL, F or ALT combinations. I'm talking pure text.
My addons include Custom New Tab, Duplicate in Tab Context Menu, IBM Notes Browser Plugin Extension, New Tab Homepage, New Tabs at the End.
After typeing the first letter of any word in a cell, the find box appears at the bottom of Firefox so the 2nd key pressed is actually entered in that, not the cell. This doesn't happen in Documents, only Spreadsheets.