This is likely due to issues with the SSL certificate issued by the server from which you are cloning the Git repository. Since you are cloning from GitHub, and I literally just went to the site, Firefox does not report the SSL certificate has issues.
Other culprits include:
- Firewalls
- General network issues
- Proxy settings
A related question has some good suggestions: 'cannot open git-upload-pack' error in Eclipse when cloning or pushing git repository. There is currently no accepted answer for that question. The error you are getting can have a number of causes, but they all appear to be network related.
Make sure you can browse to the repository on GitHub in a normal web browser. If you can't, or the browser reports SSL certificate issues, then that could be an indication this is an SSL certificate problem.