When you open the Find toolbar (shortcut Ctrl + F), for some reason there is no "Whole words only" option—this is only available in the "Find and Replace" dialogue.
It's difficult to find any rationale for this difference. It reduces the utility of the Find command, and there is plenty of space in the toolbar for the extra option.
See bug 129469 comment 10 why we resolved this RFE as wontfix.
*** This bug has been marked as a duplicate of bug 129469 ***