When I click on my Firefox web browser, I can open one window and one window only. The + sign allowing me to open and keep open multiple windows at the same time is gone. For instance, when I open my email, that works fine, but if an email contains a link to a news story I want to read and I click on it, the browser takes me there, but it closes my email window and I can't get back to it, unless I close Firefox and start over.