Checking all Mailfolders by default in Thunderbird

[Roger Grosswiler]

Hi list,

i thought, setting
user_pref("mail.check_all_imap_folders_for_new", true);

in ~/.thunderbird/*default/prefs.js

makes checking all folders for new mail. In fact it does. But once, it 
is set and you restart next time - above statement is overwritten.

Does Thunderbird copy on each start a new prefs.js in users-homedir? 
Where would i need in this case to change?

Thanks,
Roger