You actually got me thinking about it, because I've been living with a lot of this trash for so long now that I think I've probably forgotten a bunch of my gripes with thunderbird.
So here are the ones that spring to mind when I gave it a little bit of thought. I'm sure there are others that I've forgotten about because I've adopted new workflows that don't involve thunderbird (e.g my calendar is a bit like that, but I remember it because I feel like an email client with a calendar should probably be able to sync with my caldav server, and because of the stupidity of the bug). I'm also sure that as soon as I hit 'post' I'll think of more (edit: this totally happened).
* Searching IMAP folders. Worked just fine in 2010, does nothing now, no matter how long you wait. These days I just grep my maildir, like it's 1975.
* Forgetting the sort order and display preferences for folders. It LOVES to do this after an "upgrade". Because the 300,000 times I've previously told it not to show my 'cron' folder in threaded view isn't enough, apparently. I must want threaded view, but I'm just too stupid to realise it, and if they switch back to threaded view one more time maybe I'll just accept it and learn the new and better way because Mozilla knows best. Ditto for showing folder contents with the newest messages as the top - you know, the default and most useful sorting order for email. Nooooooooooo - thunderbird knows better! It loves to semi-randomly switch folders to "oldest messages at the top".
* Flat-out refusing to talk to certain older email servers because they're serving up SSL certificates using an algo that's old and which mozilla has decided they don't like anymore. What's that? The machine is one that you don't have control over and that's difficult to upgrade due to it being an ancient SunOS machine running software from like 2001, and that you're connecting to over a very secure VPN and which isn't publicly accessible, so it's no security risk at all? Tough shit, use an email client that isn't thunderbird, we're not going to provide a "proceed anyway" button for people who understand what they're doing, because Mozilla knows better than you.
* Hey there! I see you've repeatedly removed the garbage hamburger menu. This must have been an accident and not that you do not want and did not ask for it and will never want it under any circumstances ever due to your strongly-held opinion that a traditional hierarchical menu bar across the top of an application is a superior UI in every way and that hamburger menus are less efficient and have no place on a high-resolution desktop interface. So as part of the latest thunderbird "upgrade", I'm just going to helpfully slip that shitty hamburger menu that you've removed 10,000 times back into the toolbar where I think it should be, so it can waste some screen real-estate for something you'll never use. That should correct that oversight where you accidentally removed it 10,000 times. Glad I could help!
* Hey there! I see you've accidentally removed the shitty hamburger menu for the 50,000th time. I'm going to do you a solid and solve this problem once and for all - by simply making it not optional and not configurable anymore. The top right hand corner of your toolbar WILL be a hamburger menu now and forevermore. That should sort that problem out.
* Hey there! I notice that you like a traditional hierarchical menu bar, like computers have had since the 1980s. Unfortunately this isn't fashionable anymore and isn't great on phones, so what we're going to do on your high-res desktop machine is put a toolbar above the menu bar, creating a completely bizarre interface where the "get messages" button is above the File menu, in condradiction of 35+ years of UI conventions. We're also going to make this something that isn't configurable anymore. Sure, the UI used to be super-configurable for 20+ years, but that wasn't done with javascript, so we had to remove it. You really should just use the hamburger menu instead. We like it, you see, and we're not interested in your opinion if it's not the same as ours. Mozilla knows best, you see.
* I noticed that you don't have thunderbird's adaptive junk mail filter(tm) turned on. This must be an accident and not because you have sophisticated and extremely reliable enterprise-grade spam filtering solution set up on your server, with rules to do things like move email to a specific folder and mark it as read if it's determined to be spam. So what I'm going to do with this latest thunderbird "upgrade" is just silently enable the adaptive junk mail filter(tm), and then let that decide that about 40% of the thousands and thousands of messages in your inbox are junk, and then move them into a completely different and previously-unknown junk folder that you've never seen before. Now you might wonder "hey why has that colleague I was emailling back and forth with gone silent?" and you might check your junk folder to see if maybe your spam filter has gone haywire. But his messages (and a bunch of messages from your boss) won't be there! They'll be in the new and previously-unknown junk folder that I think spam should go into! And you can spend literally hours trying to find the email that's gone missing. As a bonus, we've also made it really difficult to find that missing email by breaking the search feature, and this new junk folder isn't in your maildir structure (or even on your server!), so you can't just grep for it. Have fun!
* OMG ALL YOUR RSS FEEDS ARE BROKEN! I tried to update them twice, and got an error! This must mean that all your RSS feeds coincidentally died at the same time, and is absolutely definitely not because your internet was down for maintenance for a couple of hours. So I'm going to do the only sensible thing - mark all your RSS feeds as broken and just stop ever trying to update them again until you manually tell me to update each and every one individually. No, I will not allow you to multi-select feeds so that you can update them all at once.
* I see that you like extensions. You have several installed and you use them and rely on them daily, and have for 20+ years. So what We're going to do is turn our extensions API into a shifting quagmire of incompatibility, such that extension authors have to jump through hoops every 25 minutes to make sure their extensions are compatible with the latest thunderbird, until most of them just give up. That way we can phase out the whole extensions thing like we did with firefox, giving you an objectively worse experience.
- Did you like your email headers taking up less than 50% of the message area and having the ability to double-click on the header area to toggle between full headers and compact headers? Didn't think so.
- Aah, you want to manually sort the folders in your inbox. Nah, that would be much to useful.
- Ha! A GUI to manage your sieve filters in a user-friendly and intuitive manner? That sounds far too XUL for our tastes! Begone!
(these are just the big-deal extensions that I still miss all the time and can remember off the top of my head, there used to be a BUNCH of others, too, that I used less frequently and would struggle to remember)
* What's that? You think that interfaces should get faster as computers increase in power? Oh, my sweet summer child, have you not heard the tale of Javascript and the melting CPU? No, see, we needed to disable all those xul extensions because it was bogging us down and making things inefficient! And what we're going to do is replace that with javascript trash, so that it's a whole new level of slow and unresponsive. Did you notice how I mentioned that parsing the calendar brings the whole of thunderbird to a grinding halt, making it totally unresponsive and unusable? Yeah, see that's because all this new code is very well-engineered and async / multithreaded, you see - it can fully utilise the power of a modern processor to do much much less in much much more time.
* I can't start thunderbird maximised. Haven't been able to for a few "upgrades" now. If thunderbird is in the maximised state when I close it, when I re-open it, it starts in the maximised state, and just gives me an empty window frame which it never draws anything in. To work around this, I have to: unmaximise, then close the window, then re-launch thunderbird, then once it launches normally I can maximise it and it will work. But I just don't bother maximising it these days, because that means I have to do the unmaximise/close/relaunch thing every time I start it. Instead these days I just leave thunderbird in an unmaximised state that's almost as big as it would be if it were maximised. Hilarious incompetence.
I told you it'd be vitriol ;)