Here's what gets me on his assertions.  He alleges he did it because of "the Russian thing" not to impede the investigation but because he didn't trust Comey to do a good enough job.  So why did he try to lie to the American people by setting up Rosenstein as the ax man?  In addition, if getting "the Russian thing" done right he could appoint a special prosecutor.  He could have tasked Rosenstein with finding the right replacement instead of fabricating a three page lie.
These two things leads me to believe both of the reasons he's offered to the American people are lies and there might just be something damning in the FBI's investigations.  Of course, it's always possible that Trump just forgot to fire him a while ago and doesn't want to fess up about being a forgetful old man.  His tendency to distrust others doesn't help.