I've been pushing Thunderbird to my hosting clients for years, and have had a near 95% success rate with those clients moving from MS Outlook/Outlook Express to Thunderbird.
However, your latest versions seems to reflect the very reasons why I made the push for my clients to use Thunderbird versus MS products - being that now it seems Thunderbird has become much more difficult to set up email accounts. Accounts now seem default to IMAP and great steps must be taken in order to set up a POP3 account.
This, frankly, sux.
Also, Thunderbird used to automatically allow ALL incoming mail be placed into a single inbox. Alas, it seems this is no more - and has become much like the bloatware from MS.
Sadly, many of my clients are looking to me for answers, and I don't have one. I can only recommend that they DO NOT upgrade their existing versions of Thunderbird.
And sadly, I can no longer recommend your product.
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CHAMP0Global Inbox is still there, but for POP accounts only.
Tools|Account Settings|<select account>|Server Settings→Advanced
Unified folders give very much the same feel, can include IMAP accounts, and combine all of the folders: sent, trash, junk etc, so is more comprehensive than Global Inbox.
Let the setup run to the end. Click Edit, set up the POP server options, job done.
And is IMAP really less suitable to your clients? Don't they need access from both PCs and mobile devices?
Maybe you should do a little more research before dismissing Thunderbird.