@-moz-document url(chrome://browser/content/browser.xul) {
    toolbarbutton#email-link-button > image.toolbarbutton-icon {
        list-style-image: url("data:image/x-icon;base64,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") !important;
        --code:((obj, btn) => {
                btn.onclick = btn.oncontextmenu = e => {
                    if (e.detail == 1) obj[e.type](e);
                }
                btn.setAttribute("label", "Reload user{Chrome, Content}.css");
                btn.setAttribute("tooltiptext", "L: Reload userChrome.css\nR: Reload userContent.css");
            })({
                click(e) {
                    if (e.button || !this.chromeSheet) return;
                    e.preventDefault();
                    this.reload(this.chromeSheet);
                    this.restyle(0);
                },
                async contextmenu(e) {
                    if (e.ctrlKey || e.shiftKey || !this.contentSheetURL) return;
                    e.preventDefault();
                    var count = Services.ppmm.childCount, one = count == 1;
                    var data = await this.reloadTab("about:config", one ? false : {});
                    if (one) this.reloadTab();
                    else if (data) {
                        var url = "data:," + encodeURIComponent(
                            this.processScript + '("' + this.contentSheetURL + '", ' + JSON.stringify(data) + ")"
                        );
                        var types = ["web", "file", "extension"];
                        for(var ind = 0; ind < count; ind++) {
                            var child = Services.ppmm.getChildAt(ind);
                            types.includes(child.remoteType) && child.loadProcessScript(url, false);
                        }
                    }
                    this.restyle(250);
                },
                processScript: `((href, data) => {
                    var en = Services.ww.getWindowEnumerator(null);
                    if (!en.hasMoreElements()) return;
                    var win = en.getNext();

                    var reload = sheet => {
                        var style = data[sheet.href]; if (!style) return;

                        win.InspectorUtils.parseStyleSheet(sheet, style);
                        for(var ind = 0, len = sheet.cssRules.length; ind < len; ind++) {
                            var rule = sheet.cssRules.item(ind);

                            rule.type == rule.IMPORT_RULE
                            && rule.styleSheet.href.startsWith("file:///")
                            && reload(rule.styleSheet);
                        }
                    }
                    var sheet = win.InspectorUtils.getAllStyleSheets(win.document).find(sheet => sheet.href == href);
                    if (sheet) reload(sheet);
                })`,

                reload(sheet, obj) {
                    var style = Cu.readUTF8URI(Services.io.newURI(sheet.href));
                    if (style.startsWith("\ufeff")) style = style.slice(1);
                    InspectorUtils.parseStyleSheet(sheet, style);
                    if (obj) obj[sheet.href] = style;
                    for(var ind = 0, len = sheet.cssRules.length; ind < len; ind++) {
                        var rule = sheet.cssRules.item(ind);

                        rule.type == rule.IMPORT_RULE
                        && rule.styleSheet.href.startsWith("file:///")
                        && this.reload(rule.styleSheet, obj);
                    }
                    return obj;
                },
                reloadTab(url, obj) {
                    var tab = gBrowser.addTab(url);
                    tab.style.setProperty("display", "none", "important");
                    return new Promise(resolve => {
                        var result, stop, destroy = () => {
                            if (!stop) resolve(result), gBrowser.removeTab(tab), stop = true;
                        }
                        setTimeout(destroy, 500);
                        try {
                            tab.linkedBrowser.addEventListener("DOMContentLoaded", e => {
                                var sheet = this.getSheet(e.target, this.contentSheetURL);
                                if (sheet) result = this.reload(sheet, obj);
                                destroy();
                            }, {once: true});
                        } catch(ex) {
                            destroy();
                        }
                    });
                },
                getSheet(doc, href) {
                    var sheets = InspectorUtils.getAllStyleSheets(doc);
                    return sheets.find(sheet => sheet.href == href);
                },
                get contentSheetURL() {
                    var file = Services.dirsvc.get("UChrm", Ci.nsIFile);
                    file.append("userContent.css");
                    if (!file.exists()) return null;
                    delete this.contentSheetURL;
                    return this.contentSheetURL = Services.io.newFileURI(file).spec;
                },
                get restyle() {
                    var sss = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Ci.nsIStyleSheetService);
                    var uri = Services.io.newURI("data:text/css,:root{}"), type = sss.USER_SHEET;
                    delete this.restyle; return this.restyle = delay => setTimeout(() => {
                        sss.loadAndRegisterSheet(uri, type);
                        sss.unregisterSheet(uri, type);
                    }, delay);
                },
                get chromeSheet() {
                    var file = Services.dirsvc.get("UChrm", Ci.nsIFile);
                    file.append("userChrome.css");
                    if (!file.exists()) return null;

                    var href = Services.io.newFileURI(file).spec;
                    var sheet = this.getSheet(document, href);
                    if (!sheet) return null;

                    delete this.chromeSheet; return this.chromeSheet = sheet;
                }
            }, this.parentNode) !important;
        -moz-binding: url('data:text/xml,\
            <bindings xmlns="http://www.mozilla.org/xbl"><binding id="user{Chrome,Content}.css_Reloader"><implementation>\%0A\
                <constructor>eval(getComputedStyle(this).getPropertyValue("--code"));</constructor>\%0A\
            </implementation></binding></bindings>\
        ') !important;
    }
}