Does this also occur when you run Thunderbird in Safe Mode ? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird Can you supply some images that show an example of the original received Plain Text email and what occurs to text when you click on 'Reply' / 'Forward' for that email? Can you confirm that received email was sent in both HTML and Plain text or only Plain Text by looking in the Source Code of email ? Can you confirm you are viewing emails in 'Plain Text' mode or something else: 'View' > 'Message Body as' > 'Plain Text' Is your mail account set up to not use HTML, so you only use Plain Text ? Right click on mail account name in Folder Pane and select 'Settings' Select 'Composing & Addressing' Is this checkbox selected: 'compose messages in HTML format' ? If it is selected: What happens when you hold down 'Shift' key and click on 'Reply', do you get a different result?