Microsoft Office XML metadata are copied in CKEditor

Issue

• Liferay content editors and administrators may sometimes notice that, on Firefox ESR, when copying (CTRL+C) and pasting (CTRL+V) the content of e.g. Microsoft Word in the CKEditor, the below XML metadata will be copied alongside and will show when switching to the source mode:
  
  

  <p><!--[if gte mso 9]><xml>
  <o:OfficeDocumentSettings>
  <o:AllowPNG/>
  </o:OfficeDocumentSettings>
  </xml><![endif]--><!--[if gte mso 9]><xml>
  <w:WordDocument>
  <w:View>Normal</w:View>
  <w:Zoom>0</w:Zoom>
  <w:TrackMoves/>
  <w:TrackFormatting/>
  <w:HyphenationZone>21</w:HyphenationZone>
  <w:PunctuationKerning/>
  <w:ValidateAgainstSchemas/>
  <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid>
  <w:IgnoreMixedContent>false</w:IgnoreMixedContent>
  <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText>
  <w:DoNotPromoteQF/>
  <w:LidThemeOther>DE-AT</w:LidThemeOther>
  <w:LidThemeAsian>X-NONE</w:LidThemeAsian>
  <w:LidThemeComplexScript>X-NONE</w:LidThemeComplexScript>
  <w:Compatibility>
  <w:BreakWrappedTables/>
  <w:SnapToGridInCell/>
  <w:WrapTextWithPunct/>
  <w:UseAsianBreakRules/>
  <w:DontGrowAutofit/>
  <w:SplitPgBreakAndParaMark/>
  <w:EnableOpenTypeKerning/>
  <w:DontFlipMirrorIndents/>
  <w:OverrideTableStyleHps/>
  </w:Compatibility>
  <m:mathPr>
  <m:mathFont m:val="Cambria Math"/>
  <m:brkBin m:val="before"/>
  <m:brkBinSub m:val="&#45;-"/>
  <m:smallFrac m:val="off"/>
  <m:dispDef/>
  <m:lMargin m:val="0"/>
  <m:rMargin m:val="0"/>
  <m:defJc m:val="centerGroup"/>
  <m:wrapIndent m:val="1440"/>
  <m:intLim m:val="subSup"/>
  <m:naryLim m:val="undOvr"/>
  </m:mathPr></w:WordDocument>
  </xml><![endif]--><!--[if gte mso 9]><xml>
  <w:LatentStyles DefLockedState="false" DefUnhideWhenUsed="false"
  ...

Environment

• Liferay DXP 7.0+
• Firefox 68.11.0esr (32-Bit) 

Resolution

• Upgrade your Firefox ESR client to the latest version