To reproduce in Thunderbird, you receive an email with HTML remote content (pictures and links) and Thunderbird will automatically block the remote content and display that warning message bar.
In Firefox, I have the NoScript add-on installed. NoScript blocks JavaScript content and displays that warning message bar.
I general, in both software, all warning messages appear like that at the top or at the bottom of the window.
To reproduce in Thunderbird, you receive an email with HTML remote content (pictures and links) and Thunderbird will automatically block the remote content and display that warning message bar.
In Firefox, I have the NoScript add-on installed. NoScript blocks JavaScript content and displays that warning message bar.
I general, in both software, all warning messages appear like that at the top or at the bottom of the window.