I have transformed the former extension to a userChrome.js script. That is the only way I found to get it working in current firefox verions. You need to install userChromeJS.js from here https://github.com/ardiman/userChrome.js (follow the steps as described there, I also needed to replace the config.js file as I have reported there in the issues section, but I'm not sure if that is needed for your setup). Then put the following code in either your userChrome.js file in your profile chrome folder or in a separate file by the name of the script an import it in your userChrome.js
Code
// ==UserScript==
// @name xClearFieldButton.uc.js
// @namespace xClearFieldButton@iwo.uc.js
// @description xClearbutton test
// @include main
// @compatibility Firefox 67-72
// @author Tobias Rupf
// @version 0.5
// @note build button to clear address bar and search bar fields for use in conjunction with middle mouse paste (about:config "middlemouse.paste"=true)
// ==/UserScript==
(function clearFieldButton() {
if (!location.toString().match('chrome://browser/content/browser\.(xul)|(xhtml)')) return;
if (document.getElementById("urlbarClearButton")) return;
function createBtn() {
var xClearBtnBox = document.createElementNS('http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul', 'hbox');
xClearBtnBox.setAttribute("class", "urlbar-icon-wrapper");
xClearBtnBox.setAttribute("onclick", "if (event.button == 0) {\
if (typeof(this.parentNode.parentNode.reset) == 'function') {\
this.parentNode.parentNode.reset();\
this.parentNode.parentNode.focus();\
} else {\
obj = this.parentNode.parentNode.children;\
for (var i=0; i<obj.length; i++) {\
if (typeof(obj[i].reset) == 'function') {\
obj[i].reset();\
obj[i].focus();\
}\
else if (typeof(obj[i].className.indexOf('input-box'>0)) || typeof(obj[i].className.indexOf('textbox'>0))) {\
if (typeof(obj[i].value) == 'string') {\
obj[i].value = '';\
obj[i].focus();\
}\
for (var j=0; j<obj[i].children.length; j++) {\
if (typeof(obj[i].children[j].value) == 'string') {\
obj[i].children[j].value = '';\
obj[i].children[j].focus();\
}\
}\
}\
}\
}\
}");
var xClearBtn = document.createElementNS('http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul', 'image');
xClearBtn.setAttribute("class", "urlbar-icon");
xClearBtn.setAttribute("role", "button");
/* KDE button style */
/*xClearBtn.style.listStyleImage = "url(data:image/png;base64,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)";*/
/* CLEAR button style */
/*xClearBtn.style.listStyleImage = "url(data:image/png;base64,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)";*/
/* XCLEAR button style */
/*xClearBtn.style.listStyleImage = "url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAABHNCSVQICAgIfAhkiAAAAAlwSFlzAAALEwAACxMBAJqcGAAAABl0RVh0U29mdHdhcmUAd3d3Lmlua3NjYXBlLm9yZ5vuPBoAAACSSURBVDjLrZPNCcAgDIW9O1PXci6P6gJOIJ48Ooa0PlBJtRVaDTwSJF9M/GFst2mtuTFGZC+zj1CJscansFLqyIkh63xRQM4MThO4Kg1FStttZ2vtAHZr4TYO5qOJMO99AxDDuiKCdiDpbhWApzHNAUM7iH3LFXyCi+K+Av0IzrnPIywf4to1Lj+kLU95y2f6Yxe9Ghv54yQyLAAAAABJRU5ErkJggg==)";*/
/* Firefox standard button style */
//xClearBtn.style.listStyleImage = url("chrome://global/skin/icons/searchfield-cancel.svg");
xClearBtnBox.setAttribute("class", xClearBtnBox.className + " textbox-search-clear");
const localeString = "de_DE";
const tooltipText = localeString.indexOf("zh") == -1?"Feldinhalt löschen":"\u6E05\u7A7A\u5404\u8F93\u5165\u680F";
xClearBtn.setAttribute("tooltiptext", tooltipText);
xClearBtnBox.appendChild(xClearBtn);
return xClearBtnBox
}
var pageActionButtonbar = document.getElementById("page-action-buttons");
var urlbarClearBtn = new createBtn();
urlbarClearBtn.id = "urlbarClearButton"
pageActionButtonbar.insertBefore(urlbarClearBtn, pageActionButtonbar.firstChild);
function addSearchbarClearButton() {
var searchbarTextBox = document.getElementsByClassName("search-go-container")[0];
var searchbarClearBtn = new createBtn();
searchbarClearBtn.id = "searchClearButton"
searchbarTextBox.insertBefore(searchbarClearBtn, searchbarTextBox.firstChild);
}
addSearchbarClearButton();
var searchbar = document.getElementById('searchbar');
var observers=new window.MutationObserver(function(mutations) {
mutations.forEach(function(mutation) {
if(mutation.addedNodes!==null) {
for (var i=0; i<mutation.addedNodes.length; i++) {
if (mutation.addedNodes[i].className=='searchbar-textbox') {
addSearchbarClearButton();
break;
}
}
}
});
});
observers.observe(searchbar, {childList: true});
function addFindbarClearButton() {
var findbarTextBox = document.getElementsByClassName("findbar-container")[0];
var findhbarClearBtn = new createBtn();
findbarClearBtn.id = "findClearButton"
findbarTextBox.insertBefore(findbarClearBtn, findbarTextBox.firstChild);
}
var findbar = document.getElementById('panel-1-1');
var observerf=new window.MutationObserver(function(mutations) {
mutations.forEach(function(mutation) {
if(mutation.addedNodes!==null) {
for (var i=0; i<mutation.addedNodes.length; i++) {
if (mutation.addedNodes[i].className=='findbar-textbox') {
addFindbarClearButton();
break;
}
}
}
});
});
observerf.observe(findbar, {childList: true});
createBtn();
})();