--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -2600,16 +2600,33 @@ MainAxisPositionTracker::
   if (mPackingSpaceRemaining < 0) {
     if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_BETWEEN) {
       mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
     } else if (mJustifyContent == NS_STYLE_JUSTIFY_SPACE_AROUND) {
       mJustifyContent = NS_STYLE_JUSTIFY_CENTER;
     }
   }
 
+  // Map 'left'/'right' to 'start'/'end'
+  if (mJustifyContent == NS_STYLE_ALIGN_LEFT ||
+      mJustifyContent == NS_STYLE_ALIGN_RIGHT) {
+    if (aAxisTracker.IsColumnOriented()) {
+      // Container's alignment axis is not parallel to the inline axis,
+      // so we map both 'left' and 'right' to 'start'.
+      mJustifyContent = NS_STYLE_ALIGN_START;
+    } else {
+      // Row-oriented, so we map 'left' and 'right' to 'start' or 'end',
+      // depending on left-to-right writing mode.
+      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
+      const bool isAlignLeft = (mJustifyContent == NS_STYLE_ALIGN_LEFT);
+      mJustifyContent = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
+                                               : NS_STYLE_ALIGN_END;
+    }
+  }
+
   // Map 'start'/'end' to 'flex-start'/'flex-end'.
   if (mJustifyContent == NS_STYLE_JUSTIFY_START) {
     mJustifyContent = NS_STYLE_JUSTIFY_FLEX_START;
   } else if (mJustifyContent == NS_STYLE_JUSTIFY_END) {
     mJustifyContent = NS_STYLE_JUSTIFY_FLEX_END;
   }
 
   // If our main axis is (internally) reversed, swap the justify-content
@@ -2623,18 +2640,16 @@ MainAxisPositionTracker::
   }
 
   // Figure out how much space we'll set aside for auto margins or
   // packing spaces, and advance past any leading packing-space.
   if (mNumAutoMarginsInMainAxis == 0 &&
       mPackingSpaceRemaining != 0 &&
       !aLine->IsEmpty()) {
     switch (mJustifyContent) {
-      case NS_STYLE_JUSTIFY_LEFT:
-      case NS_STYLE_JUSTIFY_RIGHT:
       case NS_STYLE_JUSTIFY_BASELINE:
       case NS_STYLE_JUSTIFY_LAST_BASELINE:
       case NS_STYLE_JUSTIFY_SPACE_EVENLY:
         NS_WARNING("NYI: justify-content:left/right/baseline/last-baseline/space-evenly");
         MOZ_FALLTHROUGH;
       case NS_STYLE_JUSTIFY_FLEX_START:
         // All packing space should go at the end --> nothing to do here.
         break;
@@ -2800,16 +2815,33 @@ CrossAxisPositionTracker::
     if (mAlignContent == NS_STYLE_ALIGN_SPACE_BETWEEN ||
         mAlignContent == NS_STYLE_ALIGN_STRETCH) {
       mAlignContent = NS_STYLE_ALIGN_FLEX_START;
     } else if (mAlignContent == NS_STYLE_ALIGN_SPACE_AROUND) {
       mAlignContent = NS_STYLE_ALIGN_CENTER;
     }
   }
 
+  // Map 'left'/'right' to 'start'/'end'
+  if (mAlignContent == NS_STYLE_ALIGN_LEFT ||
+      mAlignContent == NS_STYLE_ALIGN_RIGHT) {
+    if (aAxisTracker.IsRowOriented()) {
+      // Container's alignment axis is not parallel to the inline axis,
+      // so we map both 'left' and 'right' to 'start'.
+      mAlignContent = NS_STYLE_ALIGN_START;
+    } else {
+      // Column-oriented, so we map 'left' and 'right' to 'start' or 'end',
+      // depending on left-to-right writing mode.
+      const bool isLTR = aAxisTracker.GetWritingMode().IsBidiLTR();
+      const bool isAlignLeft = (mAlignContent == NS_STYLE_ALIGN_LEFT);
+      mAlignContent = (isAlignLeft == isLTR) ? NS_STYLE_ALIGN_START
+                                             : NS_STYLE_ALIGN_END;
+    }
+  }
+
   // Map 'start'/'end' to 'flex-start'/'flex-end'.
   if (mAlignContent == NS_STYLE_ALIGN_START) {
     mAlignContent = NS_STYLE_ALIGN_FLEX_START;
   } else if (mAlignContent == NS_STYLE_ALIGN_END) {
     mAlignContent = NS_STYLE_ALIGN_FLEX_END;
   }
 
   // If our cross axis is (internally) reversed, swap the align-content
@@ -2821,18 +2853,16 @@ CrossAxisPositionTracker::
       mAlignContent = NS_STYLE_ALIGN_FLEX_START;
     }
   }
 
   // Figure out how much space we'll set aside for packing spaces, and advance
   // past any leading packing-space.
   if (mPackingSpaceRemaining != 0) {
     switch (mAlignContent) {
-      case NS_STYLE_JUSTIFY_LEFT:
-      case NS_STYLE_JUSTIFY_RIGHT:
       case NS_STYLE_ALIGN_SELF_START:
       case NS_STYLE_ALIGN_SELF_END:
       case NS_STYLE_ALIGN_SPACE_EVENLY:
       case NS_STYLE_ALIGN_BASELINE:
       case NS_STYLE_ALIGN_LAST_BASELINE:
         NS_WARNING("NYI: align-self:left/right/self-start/self-end/space-evenly/baseline/last-baseline");
         MOZ_FALLTHROUGH;
       case NS_STYLE_ALIGN_FLEX_START: