Add Mojeek to Pale Moon
From Pale Moon version v31.1.0
Mojeek is included as a part of the preselected list from this version of Pale Moon, you can read about this decision in their Release Notes.
For versions of Pale Moon before v31.1.0
Navigate to https://www.mojeek.com.
Click the dropdown in the search box.
Click Add Mojeek.