Hi. First, resetting your modem would not stop Firefox from opening, it doesn't require the internet to open. Secondly as it seems you are using Windows, you don't say which or which FF verison. If FF doesn't open it is a Win issue, either poor or corrupt install or poor path from however you choose to access. Further you also do not say how you try to open or what options you have tried. One possible option is to [Restart] Win, '''Do not ''' use the [Shut Down] option as this keeps old parameters in memory. If you use an icon or menu option hover and check the path, then via win explorer see if that path exists.