Adding more than three recipients to an email in Thunderbird
Solution 1:
Simply add them as a list, seperated with commas:
[email protected], [email protected], [email protected], "John Doe" <[email protected]>
Solution 2:
Just keep hitting enter after each one, and it should give you more spaces for extra entries (tested on Thunderbird 3.1.11). Note the difference in the scrollbars on each image, taken from a single email to be sent:
Commas work, too, as @trurl said already.