Gmail; setting up in Thunderbird problems

[#1]

Actually it's a clone of Thunderbird, Fossamail.
The problem has been that I've had messages about certificates such as this.
Reading round, it seems that the problem may be related to whether incoming mail is POP or IMAP. I think that wayback my gmail was set up POP, but now Gmail default is IMAP.
My incoming server settings are now showing as imap.plus.net but I am now getting warning

Sending of password did not succeed. Mail server imap.googlemail.com responded: Your account is not enabled for POP access. Please visit your Gmail settings page and enable your account for POP access.

I do not want POP access. Please; can anyone help on this one?

[#2]

Cured by creating a new FM account for Gmail.

[#3]

Yes, IIRC the only way of changing an account from POP to IMAP inside thunderbird, is to add another account with the right connection type, e.g. thruderbird doesn't allow you to change the connection type of an existing account.
This does actually kinda make sense, the files on imap are saved on a remote server, whereas on pop you keep them on the local machine. Means that the two need setting up in different ways, and easiest way of doing that is just to add additional acounts ...