Currently there is no preference for mail/news folders (with threaded view) with which you could control if the threads are initaly collapsed or expanded when you visit a specific folder. There should be a general setting that would be in efect for all folders except for those that have the view set specificaly (specific setting could be in folder properties dialog for example, just like for specific folder character encoding).

I've posted a request to the "Mozilla Thunderbird Support" forum to add this preference.

MASS-CHANGE: This bug report is registered in the SeaMonkey product, but has been without a comment since the inception of the SeaMonkey project. This means that it was logged against the old Mozilla suite and we cannot determine that it's still valid for the current SeaMonkey suite. Because of this, we are setting it to an UNCONFIRMED state. If you can confirm that this report still applies to current SeaMonkey 2.x nightly builds, please set it back to the NEW state along with a comment on how you reproduced it on what Build ID, or if it's an enhancement request, why it's still worth implementing and in what way. If you can confirm that the report doesn't apply to current SeaMonkey 2.x nightly builds, please set it to the appropriate RESOLVED state (WORKSFORME, INVALID, WONTFIX, or similar). If no action happens within the next few months, we move this bug report to an EXPIRED state. Query tag for this change: mass-UNCONFIRM-20090614

MASS-CHANGE: This bug report is registered in the SeaMonkey product, but still has no comment since the inception of the SeaMonkey project 5 years ago. Because of this, we're resolving the bug as EXPIRED. If you still can reproduce the bug on SeaMonkey 2 or otherwise think it's still valid, please REOPEN it and if it is a platform or toolkit issue, move it to the according component. Query tag for this change: EXPIRED-20100420

Still worth considering.