--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -5795,20 +5795,22 @@ nsTextFrame::ComputeSelectionUnderlineHe
       return aFontMetrics.underlineSize;
     case SelectionType::eSpellCheck: {
       // The thickness of the spellchecker underline shouldn't honor the font
       // metrics.  It should be constant pixels value which is decided from the
       // default font size.  Note that if the actual font size is smaller than
       // the default font size, we should use the actual font size because the
       // computed value from the default font size can be too thick for the
       // current font size.
-      int32_t defaultFontSize =
-        aPresContext->AppUnitsToDevPixels(nsStyleFont(aPresContext).mFont.size);
-      gfxFloat fontSize = std::min(gfxFloat(defaultFontSize),
-                                 aFontMetrics.emHeight);
+      nscoord defaultFontSize = aPresContext->GetDefaultFont(
+          kPresContext_DefaultVariableFont_ID, nullptr)->size;
+      int32_t zoomedFontSize = aPresContext->AppUnitsToDevPixels(
+          nsStyleFont::ZoomText(aPresContext, defaultFontSize));
+      gfxFloat fontSize = std::min(gfxFloat(zoomedFontSize),
+                                   aFontMetrics.emHeight);
       fontSize = std::max(fontSize, 1.0);
       return ceil(fontSize / 20);
     }
     default:
       NS_WARNING("Requested underline style is not valid");
       return aFontMetrics.underlineSize;
   }
 }