Bug 1633974, don't reset findbar when switching the findbar to a different browser, r=mikedeboer

Differential Revision: https://phabricator.services.mozilla.com/D74844

toolkit/content/widgets/findbar.js

@@ -339,8 +339,6 @@
         setFindbarInActor(this._browser, this);
 
         this._browser.finder.addResultListener(this);
-
-        this._findField.value = this._browser._lastSearchString;
       }
       return val;
     }