Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1b4pre) Gecko/20090402 Shiretoko/3.5b4pre (.NET CLR 3.5.30729) - Build ID: 20090402045633
New, odd problem: With this release, I have found that I need to disable both TMP and the Print Hint extension in order for Private Browsing to work.
With Print Hint disabled and with TMP enabled, I do get the pop-up asking if I want to use Private Browsing, but then the first tab never loads, so I suspect that this part of the problem is related to the "Restore visible tabs first" problem; I don't recall having tried using Private Browsing yesterday, so don't know for certain if that part of the problem started with yesterday's Shiretoko update.
I do know, however, that Private Browsing worked at least two days ago, so the Print Hint part of the problem seems to be related to either yesterday's or today's Shiretoko update.
Print Hint is at http://www.ruadesign.org/firefox/
That oddity found, I then found that Shiretoko does not want to enter my user name for logging on to TMP automatically. I've not tried to track down what might be causing that.
(As I am a retired fire chief, I guess that Private Browsing is below my rank...).