IMAP cached connections

[Flitsch]

This is a really annoying problem with the cached connections setting for my IMAP accounts. No matter what value I use in the accounts settings dialog or in the prefs.js, Thunderbird seems to ignore it.
Sometimes it sets cached connections back to 2 or even 5. Is there a special trick to avoid this? I even write protected prefs.js, but it didn't help.