I repost in that topic as I had similar issues today in my 0.7.2 and Firefox 11.
I thought they were gone since I applied 0.7.2 but yesterday and today had this happen to me.

session log:

[24-Apr-2012 14:27:13 +0200]: Aborted session 3aq11l9co39l0k5ohvk376hub5; no valid session data found
[24-Apr-2012 14:31:36 +0200]: Session authentication failed for vre79dtcukq4oms2tgvfnbb3r4; invalid auth cookie sent
[24-Apr-2012 15:02:19 +0200]: Aborted session fdsdtsaa4rpdngfpabcumm0mb6; no valid session data found
[24-Apr-2012 15:58:13 +0200]: Session authentication failed for fil7tqlhb9q2aqumeksou5dl95; invalid auth cookie sent
[24-Apr-2012 17:17:29 +0200]: Session authentication failed for 0lplmsk91a322vrmeq6huroib5; invalid auth cookie sent
[24-Apr-2012 17:58:24 +0200]: Session authentication failed for jc708uauhjs8top5o7a4fokec7; invalid auth cookie sent

SQL log for 17:58:

[24-Apr-2012 17:58:24 +0200]: query(1): SELECT vars, ip, changed FROM session WHERE sess_id = 'jc708uauhjs8top5o7a4fokec7';
[24-Apr-2012 17:58:24 +0200]: query(1): SELECT * FROM users WHERE user_id = '151';
[24-Apr-2012 17:58:24 +0200]: query(1): DELETE FROM session WHERE sess_id = 'jc708uauhjs8top5o7a4fokec7';
[24-Apr-2012 17:58:26 +0200]: query(1): INSERT INTO session (sess_id, vars, ip, created, changed) VALUES ('jc708uauhjs8top5o7a4fokec7', 'bGFuZ3VhZ2V8czo1OiJkZV9ERSI7dGVtcHxiOjE7', 'xxx.xxx.xxx.xxx', '2012-04-24 17:58:26', '2012-04-24 17:58:26');

Is it normal, that the session gets deleted (DELETE FROM) and then INSERTed again?

Will look closely and follow all logs when it happens next time. It's very random and can't be reproduced by doing some steps.