When I open Firefox, I see an emtpy tab briefly opening beside the tabs from my previous session. The additional spurious tab is closed immediately, but not fast enough to make it unnoticeable. This is a recent regression. It's easily visible if you only have a few tabs in your session.
I can't reproduce this anymore. It must be intermittent.
I can reproduce again, so yes, it's definitely intermittent.
Strangely, at the moment I can reproduce the bug consistently, but I cannot reproduce it with mozregression (even if I use the same profile). Also, I've noticed that when I start Firefox a "about:home" page is loaded and quickly replaced with the first tab of my previous session (I have two tabs in the session). If I run Firefox via mozregression, I don't see the "about:home" page briefly appearing and the correct tab (the second one) is focused since the beginning.
I see this problem in the recent builds.