C# search query with linq

I think you just have it backwards:

mycontext.persons
    .Where(t => 
        t.Firstname.Contains(search) || 
        t.Lastname.Contains(search) || 
        t.Description.Contains(search))
    .ToList();

One possible (but probably not the most optimized solution) would be to append all of your fields together and do a Contains on the search term., e.g.

var result = persons.Where(q => (q.Description + " " q.FirstName + " " q.LastName)
                    .ToLower()
                    .Contains(searchTerm.ToLower()))
                    .ToList();