My question to you is why post your feedback in a peer support forum? Just be aware that the developers and the paid employees tend not to appear here at all. You get peers in a peer support forum. Certainly not paid developers.

There is a feedback link on the Thunderbird help menu, along with the link to this forum.

- multiple folders can't be selected in the same time

Why would you want to select multiple folders at the same time? I honestly can not think of a single reason to select more than one folder at a time. Given drag and drop and most functions ignored any folder after the first anyway and yet others simply messed up I am rather glad multiple selections are missing. Folk thought the folder pane should work like windows file explorer, and it never did.

- Customized icon function also has been removed

Customized icon how? Application? Folder? I do not recall being able to modify folder icons ever. But please expand on this so I can understand.

- TB always makes folders' file corrupted, folders always must be repaired time to time

No idea what you are talking about. I have had folder corruption maybe a dozen times in as many years and never since I got rid of Third party antivirus products and bought an SSD drive. But it would underline your complaints about correspondents and threads as the rebuild/reindex process does not have regard to those personalizations.

- TB time to time gets stuck during email sending and when I restart TB it removes the email from the Draft so I lose my already written email

Stuck how. Either the mail sends via the server or it reports an error is my experience over the past decade or so. It does not just hang there like a lost child at a fair. But as I said I don't have third party antivirus which appears to contribute quite a bit to that sort of issue. Bringing all sorts of issues, including self signed encryption certificates to the table that sometime simply prevent a stable connection. You could log the issue to determine exactly what the delay is. https://wiki.mozilla.org/MailNews:Logging

- TB always turns on f*cking Threads view on all news folder but Thread view is disabled

Disabled how? Did you follow the instructions here https://support.mozilla.org/en-US/kb/message-threading-thunderbird#w_changing-default-threading-mode I find the threading, like the correspondents column tend to reset if the actual default is not changed. Especially in folder repairs.

- TB always uses Correspondents column instead of "From", "Recipient" and "Size", I can't set a customized default view.

That is truly painful, but it has been going on ever since they invented the correspondent column. Periodically I will notice my inbox has no column for sender. But it is hardly something that can be parked at the door of supernova. The inability to resize the filters dialog. That appear to be a supernova issue.

My understanding is using the config editor to set mailnews.ui.upgrade.correspondents set to false. will prevent the insertion of the column the first time you select the folder. But that was 8 years ago, so it might not work that way now. https://bugzilla.mozilla.org/show_bug.cgi?id=1152706#c40 But it does reinforce my point that the issue is not new.

- I always have to restart the TB after a few days because it becomes very slow

Have you undertaken any investigation of the issue? See https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems The discussion is memory problems, but it sounds like that is what you are talking about as slowing down is almost always related to memory. Fundamentally the diagnostic steps are mostly the same whatever issue you are trying to diagnose, outside of the logging steps.

Since Supernova has been released we almost each day got calls from our clients and complain about TB why it is getting more useless day by day. So it is a sad story but true. Thanks, Zsolt

Actually blanket it sucks stories have little or no interest to me. I am always looking for specifics and I know when filing a bug developers live on facts and steps to reproduce. So what are the issues, beyond your clients not knowing how to use the new version with it's modified layouts and user interface changes that are not really as good in my opinion as those who developed them think they are.

Did you know that less than 1% of Thunderbird users fund all the work we do? Here are three ways to increase the funding rate:

1) Make it easier to move to another email client and the rate will grow with the reduction of imprisoned users. 2) Increase the Product Market Fit and excite your current users so much that they are willing to donate. 3) Offer unique and innovative features, your community has requested many to create a freemium business model.

I'm sure there will be many other ideas within your community. PLEEEZZZZ listen to them, and your funding rate will grow exponentially.

Crossing fingers

Generally there is this boiler plate text about if the information here does not solve your issue then consider opening your own topic. But as the original poster has seen fit to abandon their apparent support request, I will lock this topic to discourage further off topic comments that belong in the feedback forum.

There is a feedback link on Thunderbird's help menu. I suggest you use it for offering feedback as your posting here is off topic.