How to clear a Firefox address bar without selecting its content?

Ctrl + L selects the Location Bar, giving it focus and highlights whatever is in the field. Simply use that keyboard shortcut just after giving the Firefox window focus but before you paste your intended target URL.

Just use Xclear add-on.

Clear url and search field easily https://addons.mozilla.org/en-US/firefox/addon/xclear/

As per my tests Alt+A is not working as intented but Ctrl+A definitely works, since "Xclear" or "Clear fields" extensions are not working anymore with Firefox 60 or later.

The diference between Ctrl+A and Alt+A is that the later puts the selection into PRIMARY buffer (overwriting your selection) while Ctrl+A does not, so you can select a text in any application, Ctrl+A to highlight any text field (not only the address bar), press Del to delete it and then paste your previous selection with middle mouse button.

Activate the address field if necessary with a click, Select the url with Alt-A, clear the url with Del, and paste the new url with middle mouse button click.

It is Ctrl + K to select the additional address bar search box (and highlight without over-riding the Primary clipboard).

I have made a userChromeJS script which will add clear buttons to the urlbar and findbar.

// ==UserScript==
// @name        xClearFieldButton.uc.js
// @namespace       [email protected]
// @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();
})();