Are these perhaps RSS feeds and not email? There is a setting to allow RSS messages to be shown either as messages or as Web pages. But I don't think I have ever seen what you're reporting in email messages.

Hello, thank you for reply. Indeed it happens with feeds. [Edited ... removed unnecessary info]

You say : Double click on the message

A single left click (for right handed person) should open it in the 'Message Pane' assuming the Message Is enabled.

What happens for a single click ?

Info: https://support.mozilla.org/en-US/kb/how-subscribe-news-feeds-and-blogs quote: 5. You may also select the view action to perform when double clicking or hitting <enter> on a feed message selected in the list. The options are found in Message > When Opening Feed Messages.

Try setting prefered options here: Message > When Opening Feed Messages

ok, thank you all for replies! I have found that rss.show.content-base = 1 does the trick.