If chrome creates a blank content document (e.g. by opening a tab) then the resulting document has a base URI equal to the chrome document's URI. This was unexpected to me (I accidentally tried to use the base for a relative URI). Note that if content creates a content document by calling window.open() then the resulting document has an about:blank base URI as expected. Note that the editor also manages to reset the base URI to about:blank (testing at two stages of message compose shows the compose window as the initial base URI early on but later on it is about:blank as expected.) Gecko 2 has the bug but I found a build from 2007 that does not.
See bug 610001.