Hi

What do you mean by won't install? Are you trying to get to the DuckDuckGo search page?

This will hopefully help me to find a solution for you.

There isn't anything to install in Firefox for Android relating to Duckduckgo. You can install an add-on in your Firefox Android app, however, I'm not certain that will solve your problem.

I would suggest that you set https://duckduckgo.com/ as your homepage so when you start a new tab it takes you right to DuckDuckGo - no additional installation necessary.