I just checked the related code. This is obviously because all those tags except <base> are special cased to be displayed normally instead of being dimmed.
See
https://github.com/firebug/firebug/blob/master/extension/content/firebug/lib/xml.js#L579-L639.
So, it looks like the <base> tag simply got missed when the list of exception tags got created.
Note that the Firefox DevTools do a better job here. They don't have those exceptions and display all those tags slightly faint.
And as Firebug 3 will integrate into the DevTools, it will be fixed once that version is released.
Sebastian