It would have been helpful if you had made the wizard work better before asking for reviews. For example, the Previous button does not actually return to your previous choices, and the Install buttons do not actually install anything. There's no Finish button, and the whole thing appears to come to a dead end.

The pages are completely independent, so the Previous/Next wizard paradigm is meaningless. A simple tabbed UI would make more sense.

Disk Space
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The "Disk space" page does not explain what the settings have to do with disk space.

The term "synchronize" is not explained — synchronize what with what?

The wizard implies that storing messages offline is a new feature of Thunderbird 3, but it is not.

The explanations are pale grey, making them look like disabled options, and also making them difficult to read on some screens.

The text of each radio button is designed to look like a web link, with underline and a different mouse pointer, but clicking on them does not link anywhere.

It is not clear how per-account synchronization works. Does choosing that actually do anything?

I have only one IMAP account in this profile. Why am I offered "per-account synchronization" at all?

The page tells me I am "running low on space" and have 0.0 MB free space. (In fact my IMAP server is down.)

It tells me my "current total email usage" is 0.0 MB, when in fact I have mail in POP3 accounts.

New Message Toolbar
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There is no picture of the original toolbar.

The claim, "All your messaging buttons at your finger tips" is false. The new toolbar only supports some of the buttons that act on messages, not all of them. The reference to "finger tips" is completely meaningless.

The claim "Large buttons across the top of the window" is false. Mine are the same size as the new buttons in the message header.

Collapsed headers
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The claim "In order to treamline [sic] the Thunderbird experience" is false. There is no good reason that a compact version could not be incorporated in the new header design.

Extra Folder Columns
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You can tell from my profile whether or not I use the extra columns, so why do you have to ask here?

A cool improvement would be to give the user an option to just see the message and don't download the attachments. When working with IMAP and dealing with big attachments, sometimes it's harsh to wait for the entire message to download to be able to see it. Nice work, everybody :)

Migration is taking longer than 15 min.! ??
Last upgrade seemed to cause long delays in getting, deleting, responding to and opening mail files, sometimes requiring shut down and reboot several times per use and that not always fixing problem...