Нигде не встречал такую работу с ресурсами:
QUrl( "qrc:/input.html" );
знаю только такой прием:
QUrl( ":/input.html" );
Цитата из документации по QTextBrowser:
Navigation
QTextBrowser provides backward() and forward() slots which you can use to implement Back and Forward buttons. The home() slot sets the text to the very first document displayed. The anchorClicked() signal is emitted when the user clicks an anchor. To override the default navigation behavior of the browser, call the setSource() function to supply new document text in a slot connected to this signal.
If you want to load documents stored in the Qt resource system use qrc as the scheme in the URL to load. For example, for the document resource path :/docs/index.html use qrc:/docs/index.html as the URL with setSource().