Safari on MBP keeps running extensions for the 'first' time on startup
Soon after writing the question, I realized that even with no plugins installed, I had the Welcome page appear, which changed what I googled for. That helped me find:

https://discussions.apple.com/thread/6663182?start=0&amp;tstart=0

sudo find ~ $TMPDIR.. -exec chflags -h nouchg,nouappnd,noschg,nosappnd {} + -exec chown -h $UID {} + -exec chmod +rw {} + -exec chmod -h -N {} + -type d -exec chmod -h +x {} + 2&gt;&amp;-