var id = snapshot.val().id;
    var content = snapshot.val().content;
    var textObj = '<p class="task-item'+id+'">\
                    <input type="checkbox" />\
                    <span class="ui-icon ui-icon-clock"></span>\
                    <span class="task-content">'+content+'</span>\
                    <span class="task-detail">&nbspdetail</span>\
                    </p>';

The essence of your question is how to change the spliced ​​HTML template. The essence is to splice strings.
The stupidest method

var textObj1 = '<p class="task-item"';
var textObj2 = '>\
                    <input type="checkbox" />\
                    <span class="ui-icon ui-icon-clock"></span>\
                    <span class="task-content">'+content+'</span>\
                    <span class="task-detail">&nbspdetail</span>\
                    </p>';
textObj = textObj1 + id + textObj2

Or use ES6 string template directly, which is more in line with your variable nesting. But there will be compatibility issues.

var textObj = `<p class="task-item${id}">\
                        <input type="checkbox" />\
                        <span class="ui-icon ui-icon-clock"></span>\
                        <span class="task-content">'+content+'</span>\
                        <span class="task-detail">&nbspdetail</span>\
                        </p>`;

Since you are actually concatenating strings with JS, you can directly use JS’s + to concatenate the strings.
Now that you know how to splice content, the same goes for splicing id

ES5

var id = snapshot.val().id;
var content = snapshot.val().content;
var textObj = '<p class="task-item' + id + '">\
    <input type="checkbox" />\
    <span class="ui-icon ui-icon-clock"></span>\
    <span class="task-content">' + content + '</span>\
    <span class="task-detail">&nbspdetail</span>\
</p>';

Of course, you can also use ES6 template strings, but there will be issues with browser compatibility, as follows: (Note that the entire textObj starts with ` and ends with `)

ES6

var id = snapshot.val().id;
var content = snapshot.val().content;
var textObj = `<p class="task-item${id}">\
    <input type="checkbox" />\
    <span class="ui-icon ui-icon-clock"></span>\
    <span class="task-content">${content}</span>\
    <span class="task-detail">&nbspdetail</span>\
</p>`;

Addattribute try it