I have a problem with Thunderbird (currently version 17.0) on LM14 which was also apparent on LM13 in later days before I updated. When updating I transferred the Tbird profile folder so it is understandable it continued.
The problem is that when I try to forward an HTML composed email, such as a newsletter or advertisement, all the HTML is stripped out and just complex text is displayed. Edit>Preferences>Composition>Send Options>Text Format is set to sending both plain text and HTML as it always has been for all the years I have used Mint/Thunderbird.
I do have the OpenPGP encryption facility extension for occasional use but the problem developed some fair time after adding it. I am assured by other users of OpenPGP that it is not relevant to this forwarding problem. I am reluctant to remove OpenPGP just to verify that because I have forgotten the complex procedure to set it up after installing but, as I said, the problem developed later.