John Navas wrote:

One of my clients who uses Thunderbird for email has repeatedly
complained that the default email client is getting changed to Outlook
(2003).
....snip...
However, I finally narrowed the problem down to Office Update --
whenever an Outlook update is applied, or possibly even an Office
update, the update process changes the default email program from
Thunderbird to Outlook.

I don't think that's all that unusual. I've seen it happen before and it's
not a surprise to me. Simply resetting the default mail program (and
possibly repairing the URL file types) should put things back to what you
want.
--
Brian Tillman [MVP-Outlook]