Bugs, bugs, bugs, ...
Let's start with the bad news. At time of writing, 20th August 2021, Bugzilla shows 14.312 open "tickets" in Thunderbird (components Thunderbird: 7828, Mailnews Core: 4257, Calendar: 1996, Chat Core: 231, Bugzilla can't retrieve more than 10.000 bugs, you need to query separately), not counting the bugs in the Mozilla platform code that affect Thunderbird (like in the Editor, Spellchecker, Serialiser, etc.). Not all "tickets" are bugs, of course, but a fair portion is. Chances are that you are experiencing one of those.
We are here to help
So what do to? In general, we're here to help, but as you will understand, our small team cannot fix all 10.000+ bugs given that some of them are two decades old and the Thunderbird team which has many more resources than we do haven't addressed those bugs.
As a first step, we suggest to install the current ESR 91 version of Thunderbird in parallel and see whether the issue exists there. If so, it's a good idea to check whether a bug has been reported already at Bugzilla. If the issue exists in Thunderbird, regardless of whether it has been reported at or not, you can contact us and we can check how hard it would be to fix the issue. We may also advise you to file a bug in Bugzilla, so we can offer our fix to upstream Thunderbird, or we may chose to file the issue ourselves. Generally the idea of Betterbird is that our team tries to provide users with a better experience and that involves fixing long-standing and annoying bugs that somehow the Thunderbird team is not addressing.
In any case, common sense bug reporting rules apply, so something that's not reproducible won't be fixed and the user needs to cooperate during the debugging phase by following our suggestions and providing feedback or debugging information.
How to switch between Thunderbird and Betterbird?
Betterbird is designed to be 100% compatible with Thunderbird of the same version. For example at time of writing you can switch between Betterbird 91.3.2-bb21 and Thunderbird 91.3.2 without problem. If Betterbird doesn't already detect your Thunderbird profile, start Betterbird with the -p option which will launch the profile manager. You can select your regular profile there. Another option is to view all profiles via Help > More Troubleshooting Information. Click on the about:profiles link which will take you to a tab where you can select a different default profile.
When the multi-line view has been used in Betterbird, you need to switch the Subject column on again in the column picker before switching back to Thunderbird.
What's the story with the header pane buttons?
By popular demand, Betterbird has restored the header pane button customisation. We've received reports that all header pane buttons had disappeared (like "configured away") for a few users. To restore the buttons, start Betterbird in so-called Troubleshoot Mode (Help > Troubleshoot Mode) and when Betterbird restarts, select "Reset toolbars and controls".
Also note that the add-on "Message Header Toolbar Customize" is not working in Betterbird.
What's the story with complex search terms in filters, folder search and virtual/search folders (from 91.9.0-bb31)?
By popular demand, Betterbird implemented complex search terms starting in Betterbird 91.9.0-bb31. To implement that, we had to change the syntax of the content of the msgFilterRules.dat file that stores the rules a little bit while staying 100% compatible with Thunderbird as long as no complex search terms are used. In other words: If you only use simple search, you can switch between Thunderbird and Betterbird with no problem. If you use complex search terms, opening the Message Filter panel (Tools > Message Filters) will damage the complex filters set up in Betterbird. Just starting Thunderbird won't do any damage. If you set up search folders using complex search terms, they won't work in Thunderbird.
XML Parsing Error: undefined entity. What's going on?
If you receive a XML Parsing Error: undefined entity error (in German: XML-Verarbeitungsfehler: Nicht definierte Entität) when your start Betterbird, you have a Thunderbird language pack installed. All Thunderbird language packs are incompatible with Betterbird. Please remove them. You can either switch to Thunderbird and remove them via Tools > Add-ons and Themes, then click on the Languages tab. Or you can remove them directly from the extensions folder in your profile. The filename starts with langpack. German users can follow this article or this article.
Is Betterbird a virus?
We have seen cases where Windows Security makes you believe that Betterbird contains malicious code, we got a report for "Trojan:Script/Wacatac.B!ml" with the wording "This program is dangerous and executes commands from an attacker". Scary stuff. We've since started self-signing all executables, you can download our certificate which you need to install into "Trusted Root Certification Authorities" (check with certmgr.msc). The result looks like this. We've also excluded the Betterbird program directory and process from further scans.
Betterbird doesn't contain any more malicious code than the Thunderbird or Mozilla codebase, but we admit that we haven't inspected the millions of lines of upstream code. The binaries we distribute are built from open source code and you can build the package yourself using the instruction on the Github repository. You can also submit alleged malicious code to an online virus scanner like VirusTotal or Jotti.
One word of caution: E-Mail in general is dangerous. Like all written communication, it doesn't convey any facial expression, tone of voice and irony is frequently misunderstood. So use e-mail wisely and don't pick a fight! Also, e-mail is dangerous when sending the wrong stuff to the wrong people. Please check twice to whom you're sending the message before clicking "Send". We speak from experience :-( - If you're transmitting confidential information, you should consider setting up OpenPGP or using p≡p
Lastly and humorously: Some consider the Windows operating system to be a boot sector virus. We hope Betterbird will spread like a virus, and we need you to make the project a success.