Comment 18 for bug 12097

I am also experiencing this bug in Karmic with updated thunderbird and firefox. I discovered that the problem is that I am running firefox with "--no-remote" option. It seems that this enables the environment variable MOZ_NO_REMOTE, which affects thunderbird also. Running thunderbird with "env -u MOZ_NO_REMOTE" solves the bug for me.
Actually, I find quite strange that this env variable is common to both programs, since they are completely independent processes and have independent profiles.