I\'ll try to make a brief description of my Selenium framework so that I can explain my problem.
I use Selenium 2 (current version 2.3.1) + test
FYI, you can output the thread id using %thread as described in the documentation at http://logback.qos.ch/manual/configuration.html :
The output is formatted using a PatternLayoutEncoder set to the pattern %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n.