This is a really old script to bookmark art in Pixiv.net using ajax in a simple click, like Deviantart, for some reason it works just fine in Chromium based browsers+Tampermonkey, but the same script doest work in Firefox 34-35+GreaseMonkey, I really appreciate if someone can modify and improve the script for use in Firefox and all other browsers, it is strange that no one has put this kind of function in a script for pixiv. Thanks in advance.

// ==UserScript==
// @name           Pixiv AJAX bookmarking
// @namespace      RevPsych
// @description    Allows bookmarking images on Pixiv without leaving the image page.
// @include        *pixiv.net/*
// @version            0.2
// ==/UserScript==

/*
BEGIN OPTIONS
*/

/*
defaultPrivacy

Set to "0" for public bookmarks of non-R18 images. Set to "1" to always set to private
*/
defaultPrivacy = "0";

/*END OPTIONS*/




/*BEGIN CODE*/

//Find the bookmark button
el = document.getElementsByClassName("bookmark-container")[0];

//Because the manga page does things differently
if(el == null){
    //Get the illustration ID#
    el = document.getElementsByName("illust_id")[0];
    illustId = el.value;

    //Create the url to the bookmark_add page
    url = "http://www.pixiv.net/bookmark_add.php?type=illust&illust_id=" + illustId;

    //Get the text from the button
    inner = el.parentNode.childNodes[2].value;

    //Select the section
    el = el.parentNode.parentNode;

    //Remove the button
    el.removeChild(el.childNodes[0]);
}
else{
    el = document.getElementsByClassName("bookmark-container")[0].getElementsByClassName("_button")[0];

    //Copy the url and text
    url = el.href;
    inner = el.innerHTML;

    //Remove the button
    el.parentNode.innerHTML="";

    el = document.getElementsByClassName("bookmark-container")[0];
}

//Create a new button
z = document.createElement("p");
z.setAttribute("class", "_button");
z.innerHTML = inner;
z.setAttribute('value', url);

//Add it to the document
z = el.appendChild(z);

//Add an event listener to launch our function
z.addEventListener('click',ajaxRelated,false);

function ajaxRelated() {

// Grab the element that called this function
el = this;

//Update the innerHTML
el.innerHTML = "<b>Adding...</b>";

// get the url that's stored in the link's value attribute
url = el.getAttribute("value");

//Fetch the bookmark page to pull tags and other necessary data
GM_xmlhttpRequest({
method: "GET",
url: url,
onload: function(o) {

//Store the response
response = o.responseText;

//Create a new element to attach the response text to
temp = document.createElement('div');
temp.style.display = "none";

//Select the text between the body tags
body = response.slice(response.indexOf(">", response.indexOf('<body'))+1,response.indexOf("</body>"));

//Now put the response text into our temporary element
temp.innerHTML = body;

//Now attach it to the document so we can use it
document.body.appendChild(temp);

//Get the div element of the recommended tags section (there are two, but we only want the first one)
related = temp.getElementsByClassName('recommend-tag')[0];

//Initialize string
tags = "";

//Set privacy
r18 = defaultPrivacy;

//Get the individual elements that contain the tags
tagEl = related.getElementsByClassName('tag');

//Get the tags and store them in a string
for(i=0;i< tagEl.length;i++){
    cur = tagEl[i].getAttribute("data-tag");

    //If r-18, mark it as such
    if(defaultPrivacy == "0" && (cur.toLowerCase() == '*r-18' || cur.toLowerCase() == 'r-18')) 
        r18 = '1';

    //Add the tag to the tag list
    tags = tags + cur + ' ';
}

//Get the hidden tt value for POSTing
var tt = document.getElementsByName('tt')[0].value;

//Extract the illustration ID from the url
var patt = /(?:[0-9]{1,})$/;
var illustId = patt.exec(url);

//Submit our information to bookmark_add.php
GM_xmlhttpRequest({
method: "POST",
url: "http://www.pixiv.net/bookmark_add.php",
headers:{'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8'},
data: "type=illust&id="+illustId+"&tt="+tt+"&restrict="+r18+"&comment=&mode=add&from_sid=&submit=&tag="+tags,
onload: function(o) {
    //Remove bookmark button
    el.parentNode.removeChild(el);

    //Replace the bookmark button with the edit bookmark button
    bC = document.createElement("a");
    bC.className = "button-on";
    bC.href = url;
    bC.innerHTML = "Edit Bookmark";
    document.getElementsByClassName('bookmark-container')[0].appendChild(bC);
}
});

}
});



}

Greasemonkey is strict. You just need to @grant access to all functions you've used, add // @grant GM_xmlhttpRequest to UserMeta block

it will look like this

// ==UserScript==
// @name           Pixiv AJAX bookmarking
// @namespace      RevPsych
// @description    Allows bookmarking images on Pixiv without leaving the image page.
// @include        *pixiv.net/*
// @grant          GM_xmlhttpRequest
// @version        0.2
// ==/UserScript==