Why is Firefox not available after I reset it?

You get an error message in the area of the window where your home page usually appears, or the window doesn't even come up at all?

If you can get a window, can Firefox connect to Google (or your preferred search engine) if you try a search from the search bar?

Try to start Firefox with the about:blank page by creating a copy of the current Firefox desktop shortcut and add a space and -url "about:blank" to the command line.