Unable to get Mozilla Firefox set as my default browser

Hi

Depending on your phone's brand, but the core steps remain the same

Open your phone's Settings app. Go to Apps or Apps & Notifications.

Tap on Default apps or "Choose default apps".

Select Browser app.

Tap Firefox to select it as your new default

more info Set Firefox as your default browser on Android

Need URL for Mozilla Firefox"'

Are you actually asking how to set Firefox as a search engine?

Firefox is a web browser and not a search engine as a search engine is for example google.com. bing.com, duckduckgo.com etc.

https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android