multi-row tabs: Only hide alltabs-button if it's not removable

This means that the button will be hidden in ESR 128, but on Firefox
132+ it won't be since one can simply remove the button themselves or
move it to another toolbar etc. Fixes #407

chrome/multi-row_tabs.css

@@ -71,7 +71,7 @@ See the above repository for updates as well as full license text. */
   display: flex !important;
 }
 
-#alltabs-button,
+#alltabs-button[removable="false"],
 :root:not([customizing]) #TabsToolbar #new-tab-button,
 #tabbrowser-arrowscrollbox > spacer,
 .tabbrowser-tab::after{ display: none !important }