You cannot use DDG without a web browser like Firefox or Safari.
You have two options to get something more like what you want:
- Make a shortcut (icon) to start Firefox or Safari, then load DDG
- Make DDG your homepage, so starting Firefox or Safari automatically takes you to DDG.
For the first, you should be able to visit duckduckgo.com in your web browser of choice, then select the web address (in the box at the top of your browser) and drag it off to your desktop or task bar.
For the second, check Firefox @ Help
or drag this: https://duckduckgo.com/
to the Home button in your browser.