update navbarToolbarButtonSlider.uc.js:

don't move non-overflowable buttons to the overflow panel.
for example, the downloads button isn't supposed to overflow.
so when the slider overflows, move the overflowable buttons to the panel
and unpack the other buttons in place (where the slider was).

JS/atoolboxButton.uc.js

@@ -426,7 +426,7 @@ const toolboxButtonL10n = {
           let shortcut = hotkey ? ` (${ShortcutUtils.prettifyShortcut(hotkey)})` : "";
           toolbarbutton.label = labelString;
           label.value = labelString;
-          toolbarbutton.tooltipText = `${labelString}${shortcut}`;
+          toolbarbutton.tooltipText = labelString + shortcut;
         };
 
         // remove this window's observers when the window closes, since observers are global

JS/navbarToolbarButtonSlider.uc.js

@@ -1,6 +1,6 @@
 // ==UserScript==
 // @name           Navbar Toolbar Button Slider
-// @version        2.8.2
+// @version        2.8.3
 // @author         aminomancer
 // @homepage       https://github.com/aminomancer
 // @description    Wrap all toolbar buttons in a scrollable container. It can
@@ -342,7 +342,9 @@ class NavbarToolbarSlider {
     let overflown = this.isOverflowing;
     let array = [...this.widgets].filter(this.filterFn, this);
     if (overflown) {
-      this.wrapAll(array, this.cOverflow);
+      array.forEach(button => {
+        if (button.getAttribute("overflows") !== "false") this.cOverflow.appendChild(button);
+      });
       this.cOverflow.insertBefore(this.outer, this.cOverflow.firstElementChild);
     } else this.wrapAll(array, this.inner);
     this.outer.hidden = overflown;
@@ -351,9 +353,12 @@ class NavbarToolbarSlider {
   onWidgetOverflow(aNode, aContainer) {
     if (aNode.ownerGlobal !== window) return;
     if (aNode === this.outer && aContainer === this.cTarget) {
-      NavbarToolbarSlider.appendLoop(this.kids, this.cOverflow);
+      [...this.kids].forEach(button => {
+        if (button.getAttribute("overflows") === "false") {
+          this.cTarget.insertBefore(button, this.outer);
+        } else this.cOverflow.appendChild(button);
+      });
       this.outer.hidden = true;
-      this.reOrder();
     }
   }
   onWidgetUnderflow(aNode, aContainer) {