--- a/accessible/generic/ARIAGridAccessible.cpp
+++ b/accessible/generic/ARIAGridAccessible.cpp
@@ -65,18 +65,20 @@ ARIAGridAccessible::ColCount() const
   Accessible* row = rowIter.Next();
   if (!row)
     return 0;
 
   AccIterator cellIter(row, filters::GetCell);
   Accessible* cell = nullptr;
 
   uint32_t colCount = 0;
-  while ((cell = cellIter.Next()))
-    colCount++;
+  while ((cell = cellIter.Next())) {
+    MOZ_ASSERT(cell->IsTableCell(), "No table or grid cell!");
+    colCount += cell->AsTableCell()->ColExtent();
+  }
 
   return colCount;
 }
 
 uint32_t
 ARIAGridAccessible::RowCount()
 {
   uint32_t rowCount = 0;
@@ -597,20 +599,19 @@ ARIAGridCellAccessible::ColIdx() const
   Accessible* row = Row();
   if (!row)
     return 0;
 
   int32_t indexInRow = IndexInParent();
   uint32_t colIdx = 0;
   for (int32_t idx = 0; idx < indexInRow; idx++) {
     Accessible* cell = row->GetChildAt(idx);
-    roles::Role role = cell->Role();
-    if (role == roles::CELL || role == roles::GRID_CELL ||
-        role == roles::ROWHEADER || role == roles::COLUMNHEADER)
-      colIdx++;
+    if (cell->IsTableCell()) {
+      colIdx += cell->AsTableCell()->ColExtent();
+    }
   }
 
   return colIdx;
 }
 
 uint32_t
 ARIAGridCellAccessible::RowIdx() const
 {

--- a/accessible/generic/TableAccessible.cpp
+++ b/accessible/generic/TableAccessible.cpp
@@ -256,15 +256,17 @@ TableAccessible::RowAt(int32_t aRow)
 }
 
 Accessible*
 TableAccessible::CellInRowAt(Accessible* aRow, int32_t aColumn)
 {
   int32_t colIdx = aColumn;
 
   AccIterator cellIter(aRow, filters::GetCell);
-  Accessible* cell = cellIter.Next();
-  while (colIdx != 0 && (cell = cellIter.Next())) {
-    colIdx--;
+  Accessible* cell = nullptr;
+  
+  while (colIdx >= 0 && (cell = cellIter.Next())) {
+    MOZ_ASSERT(cell->IsTableCell(), "No table or grid cell!");
+    colIdx -= cell->AsTableCell()->ColExtent();
   }
 
   return cell;
 }

--- a/accessible/html/HTMLTableAccessible.cpp
+++ b/accessible/html/HTMLTableAccessible.cpp
@@ -633,60 +633,104 @@ HTMLTableAccessible::CellAt(uint32_t aRo
 
 int32_t
 HTMLTableAccessible::CellIndexAt(uint32_t aRowIdx, uint32_t aColIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
   if (!tableFrame)
     return -1;
 
-  return tableFrame->GetIndexByRowAndColumn(aRowIdx, aColIdx);
+  int32_t cellIndex = tableFrame->GetIndexByRowAndColumn(aRowIdx, aColIdx);
+  if (cellIndex == -1) {
+    // Sometimes, the accessible returned here is a row accessible instead of
+    // a cell accessible, for example when a cell has CSS display:block; set.
+    // In such cases, iterate through the cells in this row differently to find it.
+    nsIContent* cellContent = tableFrame->GetCellAt(aRowIdx, aColIdx);
+    Accessible* cell = mDoc->GetAccessible(cellContent);
+    if (cell && cell->IsTableRow()) {
+      return TableAccessible::CellIndexAt(aRowIdx, aColIdx);
+    }
+  }
+
+  return cellIndex;
 }
 
 int32_t
 HTMLTableAccessible::ColIndexAt(uint32_t aCellIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
   if (!tableFrame)
     return -1;
 
   int32_t rowIdx = -1, colIdx = -1;
   tableFrame->GetRowAndColumnByIndex(aCellIdx, &rowIdx, &colIdx);
+
+  if (colIdx == -1) {
+    // Sometimes, the index returned indicates that this is not a regular
+    // cell, for example when a cell has CSS display:block; set.
+    // In such cases, try the super class method to find it.
+    return TableAccessible::ColIndexAt(aCellIdx);
+  }
+
   return colIdx;
 }
 
 int32_t
 HTMLTableAccessible::RowIndexAt(uint32_t aCellIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
   if (!tableFrame)
     return -1;
 
   int32_t rowIdx = -1, colIdx = -1;
   tableFrame->GetRowAndColumnByIndex(aCellIdx, &rowIdx, &colIdx);
+
+  if (rowIdx == -1) {
+    // Sometimes, the index returned indicates that this is not a regular
+    // cell, for example when a cell has CSS display:block; set.
+    // In such cases, try the super class method to find it.
+    return TableAccessible::RowIndexAt(aCellIdx);
+  }
+
   return rowIdx;
 }
 
 void
 HTMLTableAccessible::RowAndColIndicesAt(uint32_t aCellIdx, int32_t* aRowIdx,
                                         int32_t* aColIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
-  if (tableFrame)
+  if (tableFrame) {
     tableFrame->GetRowAndColumnByIndex(aCellIdx, aRowIdx, aColIdx);
+    if (*aRowIdx == -1 || *aColIdx == -1) {
+      // Sometimes, the index returned indicates that this is not a regular
+      // cell, for example when a cell has CSS display:block; set.
+      // In such cases, try the super class method to find it.
+      TableAccessible::RowAndColIndicesAt(aCellIdx, aRowIdx, aColIdx);
+    }
+  }
 }
 
 uint32_t
 HTMLTableAccessible::ColExtentAt(uint32_t aRowIdx, uint32_t aColIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
   if (!tableFrame)
     return 0;
 
-  return tableFrame->GetEffectiveColSpanAt(aRowIdx, aColIdx);
+  uint32_t colExtent = tableFrame->GetEffectiveColSpanAt(aRowIdx, aColIdx);
+  if (colExtent == 0) {
+    nsIContent* cellContent = tableFrame->GetCellAt(aRowIdx, aColIdx);
+    Accessible* cell = mDoc->GetAccessible(cellContent);
+    if (cell && cell->IsTableRow()) {
+      return TableAccessible::ColExtentAt(aRowIdx, aColIdx);
+    }
+  }
+
+  return colExtent;
 }
 
 uint32_t
 HTMLTableAccessible::RowExtentAt(uint32_t aRowIdx, uint32_t aColIdx)
 {
   nsTableWrapperFrame* tableFrame = do_QueryFrame(mContent->GetPrimaryFrame());
   if (!tableFrame)
     return 0;

--- a/accessible/tests/mochitest/table/test_indexes_table.html
+++ b/accessible/tests/mochitest/table/test_indexes_table.html
@@ -129,16 +129,24 @@ https://bugzilla.mozilla.org/show_bug.cg
 
       // ////////////////////////////////////////////////////////////////////////
       // Table with a cell that has display: block; style
       idxes = [
         [0, 1]
       ];
       testTableIndexes("tablewithcelldisplayblock", idxes);
 
+      // ////////////////////////////////////////////////////////////////////////
+      // A table with a cell that has display: block; and a cell with colspan.
+      // This makes us fall back to the ARIAGridCellAccessible implementation.
+      idxes = [
+        [0, 0, 1]
+      ];
+      testTableIndexes("tablewithcolspanandcelldisplayblock", idxes);
+
       SimpleTest.finish();
     }
 
     SimpleTest.waitForExplicitFinish();
     addA11yLoadEvent(doTest);
   </script>
 </head>
 <body>
@@ -414,10 +422,17 @@ https://bugzilla.mozilla.org/show_bug.cg
 
   <table id="tablewithcelldisplayblock">
     <tr>
       <th>a</th>
       <td style="display: block;">b</td>
     </tr>
   </table>
 
+  <table id="tablewithcolspanandcelldisplayblock">
+    <tr>
+      <th colspan="2">a</th>
+      <td style="display: block;" >b</td>
+    </tr>
+  </table>
+
 </body>
 </html>