Do you have the correct settings for Hotmail? They should be as shown here.

If it still doesn't work, state your antivirus and VPN, if applicable, as AV often cause timeouts if they scan the TB profile folder or secure connections.

I think the problem has been caused oddly, by the Avast Antivirus software - since if I turn it off, then I can once again send emails from my Hotmail account(s) via Thunderbird. But why is Microoft Outlook not effected ?

My Thunderbird Settings have not changed and look similar to those shown in your referenced posts.

I was until recently able to send emails from my Hotmail accounts, and I can still receive emails in those accounts. I can also still post/receive emails in my gmail based accounts via Thunderbird. Its simply the Hotmail accounts that are being blocked from sending emails. If I switch to Microsoft Outlook I can both send & receive emails from the same hotmail accounts.

As I mentioned, if I turn off Avast, then I can send emails from my hotmail accounts - as I dont use Avast Firewall, perhaps it their MailSheild function thats creating this problem - but why ?

Avast/AVG are a perennial source of issues, as a search of this forum will reveal. Disable the MailShield and any other component that affects TB or the profile. It doesn't offer any essential protection, as the real-time background scan guards against opening infected attachments.

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues