Thunderbird removes leading spaces before XML/HTML tags, how can I disable that feature?

Question

The following is especially annoying on XML / HTML / XSL / etc user groups and mailing lists. When I create a new plain-text message and type the following message in Thunderbird 9.0.1:

Check this code:
<hello>
    <world />
</hello>

it will be received as

Check this code:
<hello>
<world />
</hello>

Oddly enough, this behavior doesn't happen when a sentence doesn't start with a <-symbol. In my own Sent-folder the leading spaces are removed too. When I send as HTML mail, the problem doesn't appear. Does anybody know of a way around this?

EDIT: result of extensive comment-discussion have been moved into the body of the text.