diff --git a/mozilla/layout/generic/nsFrame.cpp b/mozilla/layout/generic/nsFrame.cpp
index b2419f2e64c..3f49ec1b174 100644
--- a/mozilla/layout/generic/nsFrame.cpp
+++ b/mozilla/layout/generic/nsFrame.cpp
@@ -2866,12 +2866,12 @@ nsIFrame::InlinePrefWidthData::ForceBreak(nsIRenderingContext *aRenderingContext
     if (floats_cur > floats_done)
       floats_done = floats_cur;
 
-    currentLine += floats_done;
+    currentLine = NSCoordSaturatingAdd(currentLine, floats_done);
 
     floats.Clear();
   }
 
-  currentLine -= trailingWhitespace;
+  currentLine = NSCoordSaturatingSubtract(currentLine, trailingWhitespace, nscoord_MAX);
   prevLines = PR_MAX(prevLines, currentLine);
   currentLine = trailingWhitespace = 0;
 }
diff --git a/mozilla/layout/generic/nsTextFrameThebes.cpp b/mozilla/layout/generic/nsTextFrameThebes.cpp
index d31d911b32c..d6dd17a25f4 100644
--- a/mozilla/layout/generic/nsTextFrameThebes.cpp
+++ b/mozilla/layout/generic/nsTextFrameThebes.cpp
@@ -4933,7 +4933,7 @@ nsTextFrame::AddInlinePrefWidthForFlow(nsIRenderingContext *aRenderingContext,
     // onto one line, so just measure it
     nscoord width =
       NSToCoordCeil(mTextRun->GetAdvanceWidth(start, flowEndInTextRun - start, &provider));
-    aData->currentLine += width;
+    aData->currentLine = NSCoordSaturatingAdd(aData->currentLine, width);
 
     PRUint32 trimStart = GetEndOfTrimmedText(provider.GetFragment(), start,
                                              flowEndInTextRun, &iter);