When you popup a tab with the donation suggestion, and the user is already typing an email that they're writing, or something else, it's likely to disrupt them and cause them to annoyedly close the window.
Pop it up on boot or nothing.
This sort of thing, much like the firefox 'congratulations, you're on a new version' is very confusing to newcomers and computer illiterate types, who I work with. If you want them to stay with thunderbird, many delay the popup until after they've been using it for a month, or something.
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