Set up two Twitter accounts.

Now try to set up an RSS feed for both of your user timelines:

1) "http://twitter.com/statuses/user_timeline/[account1].rss"
2) "http://twitter.com/statuses/user_timeline/[account2].rss"

Result: It will only login to and draw in headers for the first
account. The second account, even though it shows as having a
different URL, will still draw in headers from the first account.

Now set up two similar but different e-mail addresses:

1) abcd@aaaa.com
2) abcd@aaab.com

Same result. The second e-mail address you set up will morph into the
first address. It won't even display as a different account in your
file tree column, but will show two identical accounts.

This can be easily reproduced, ad-infinitum, yet no one seems to know
how to solve it or to acknowledge it's an apparent bug. I'm therefore
not able to use Thunderbird for all of my RSS feeds or e-mail
accounts. Completely annoying.