Firefox can’t establish a connection to the server at 127.0.0.1:8080

Thanks, how do I check?

Look it seems like the issue might be related to the application not running correctly, the port being blocked, or a misconfiguration in your Ruby on Rails application. Could you provide some additional details?

• Is your Ruby on Rails application actively running when you try to access it? You can check this by looking at the output in your Cloud9 terminal.
• Is your application configured to run on port 8080? You can verify this by checking the configuration files or the command used to start the server.
• If you're using Fedora/Linux, you can check if port 8080 is already in use by another process. Run the following command in the terminal:

netstat -tuln | grep 8080

If the port is in use, you can free it up by killing the process:

sudo lsof -t -i:8080 | xargs -r sudo kill

After killing the process, try running your application again.

• Verify your security settings to ensure that they are not blocking the port used by your Ruby on Rails server. This includes firewall settings and any security groups or rules in your cloud environment. You can refer to my post on networking: Networking 101 Part 2, specifically the section about Security Groups.
• Are there any error messages in your Ruby on Rails application logs or in the browser console?

http://127.0.0.1:8080/ Not sure what inbound security group is or how to check

If your application is running on an EC2 instance, you should check the associated security group, which acts as a firewall controlling traffic to your instance. For more information, you can refer to my post on networking: Networking 101 Part 2, specifically the section about Security Groups.