This is a combo of 2 tricks.

Based off this german guide, a bit outdated and not suited for the Flatpak

Goal

a signature like

Firstname Lastname
Role
_____
<fancy logo>
Company name
Contact info including website and mail

This is probably really easy in Outlook, and kinda linux-y (bundling together parts that only make sense if you are a programmer) on Thunderbird.

1. The Sandbox

When attaching an image, Thunderbird Flatpak needs to have permanent access to that location.

By default it uses portals, so if you use “attach image” it will use some /run/doc/… folder that is gone after restarting the app.

For some reason, using the KDE Plasma Flatpak settings or Flatseal, granting the app access to a certain location, doesnt work, even if you use the real location of the image.

So instead:

  1. Create a directory in the internal Flatpak’s folder

mkdir ~/.var/app/org.mozilla.thunderbird/SIGNATURE

  1. Copy the attachment image there (company logo etc.)
  2. Copy the path to the image (for example in KDE Dolphin, no idea about GNOME)

2. The HTML Signature

Just write a new mail (Ctrl+N) and write exactly your signature in there.

You dont need the

–

As that is automatically inserted below the last line.

Add the picture, but replace the path with the real path, not the /run/doc/… one of the portal.

Then the image is inserted, you can resize it.

Now instead of sending, in the menu under “File” use “Save to…” and instead of .eml use .html.

If you want to add a clickable mail address, in the mail compose toolbar, behind the “picture icon” there is a menu, select the “link” icon.

You can add a normal http/https link there. But using mailto:name@server.com you can make it a clickable mail link!

(Whoever needs that in a mail)

3. Sandbox again.

Save that file to the same ~/.var/app/org.mozilla.thunderbird/SIGNATURE folder.

4. Account settings

Navigate to these settings, in the first page of your account, instead of writing your signature, use “use HTML file”.

Select the file or paste the exact ~/.var/app/... location in there, again, dont use the portal.

Done!

Once figured out it makes sense. That directory in the Flatpaks storage will not be deleted or interrupt anything. So this is a clean way.

flatpak remove --delete-data thunderbird would purge that entire folder and all it’s contents.