Unless you have a very good reason, and understand the limitations, don't use POP. It's essentially an obsolete technology. Some email providers require secure signon that they do not support on POP. (Some are trying for years to abandon IMAP in favour of proprietary protocols.) With IMAP you can read and reply to email on any number of devices and it's all kept in sync. Even if you only use one device now it's likely you'll use more in future, and POP is a PITA on several devices. I was a hold-out on POP for ages, but even I converted several years ago.

As for conversion from Live Mail, I have no experience of it, but CSV sounds easy enough. Try it. You can easily load the CSV file into a spreadsheet and delete superfluous columns. I suppose you don't have to convert contacts at all if you have them in some online mail system, such as Google's. You could just connect to it with something like this:
TbSync

I have my mobile contacts sync'd with a similar service, but I've never got round to connecting Thunderbird to it.

Email clients are a bit of an anachronism. Thunderbird is one of the few left. If it didn't exist, I don't think it would be invented. But it does exist and is kept updated by a very small team and many volunteers.

Now a plug:
In a few months Thunderbird will update from 68 to 78. When it does, you might find these addons useful.
Limit non-BCC recipients
Expand mailing lists

My wife maintains several U3A mailing lists and sends emails to large groups. Those are to prevent accidentally sharing everybody's email addesses, or deleting individuals from a group mailing.