--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -6068,26 +6068,29 @@
           var doPosition = this.getAttribute("overflow") == "true" &&
                            numPinned > 0;
 
           if (doPosition) {
             this.setAttribute("positionpinnedtabs", "true");
 
             let scrollButtonWidth = this.mTabstrip._scrollButtonDown.getBoundingClientRect().width;
             let paddingStart = this.mTabstrip.scrollboxPaddingStart;
+            let pinnedTabWidth;
             let width = 0;
 
             for (let i = numPinned - 1; i >= 0; i--) {
               let tab = this.childNodes[i];
-              width += tab.getBoundingClientRect().width;
+              if (!pinnedTabWidth) {
+                pinnedTabWidth = tab.getBoundingClientRect().width;
+              }
+              width += pinnedTabWidth;
               tab.style.marginInlineStart = -(width + scrollButtonWidth + paddingStart) + "px";
             }
 
             this.style.paddingInlineStart = width + paddingStart + "px";
-
           } else {
             this.removeAttribute("positionpinnedtabs");
 
             for (let i = 0; i < numPinned; i++) {
               let tab = this.childNodes[i];
               tab.style.marginInlineStart = "";
             }