I've been a HEAVY user of Thunderbird for 8 years now and I am very saddened by this decision but I do support it. :(

One part of me saw this while reading the letter: "Google Chrome is whooping our FireFox browser's ass and we need all men at their battle stations, including people from the Thunderbird team. We've been neglecting Thunderbird for so long that it's not like anyone's going to notice. After all, it took us 10 years to get Sunbird (Mozilla's Standalone Calendar Client) to version 1.0."

And the other part of me saw this while reading the letter: "The era of desktop software is coming to an end. Most people have smart phones, multiple computers, and use web-mail to keep it all in sync and accessible across all of their devices. Desktop email client users make up a small percentage of users and there's no reason for us to keep spending money and resources on something that will one day be emulated by a web interface."

=== Things Wrong With Thunderbird That Will Probably Never Get Fixed (too expensive / not worth it) ===

- The biggest problem with Thunderbird is that it tries to be a bare-bones email client with poorly integrated functionality in the form of third party add-ons. What really makes email clients shine is when a lot of usable features are tightly integrated with a very intuitive and snappy interface. Thunderbird out of the box comes with so few features that it can't compete well with web mail and when you do add on much needed features, they just get generically "bolted on" to the interface. Sometimes in ways that just seem unintuitive and backwards. And every time you update Thunderbird, ALL of your add-ons are rendered useless and you have to wait days/weeks/indefinitely for the addon to be updated. This is the biggest downfall of Thunderbird in my eyes. You'd have to redesign Thunderbird and that isn't happening, it isn't worth it.

- By default Thunderbird tries to send all my outgoing mail through 1 smtp account. This alone causes so many problems. Each email account should send emails from its own stmp. Not doing so can mark your "from" field incorrectly (has happened to me many times), trigger red flags (happened to me before) and make other email clients mark your email as "Gmail thinks this message is a scam".

- The SPAM filter in Thunderbird is A.W.F.U.L. Let me repeat that A-W-F-U-L. Despite training it for years it routinely misses the same spam, with the same title, and the same content, while sometimes marking very important emails as junk.

- The time and date selector for Lightning is just atrocious to the point where I hate having to use it. It FORCES me to set everything in military time and makes date selection more cumbersome than it needs to be.

- The tasks todo list for Lightning has never worked for me. Never.

- Thunderbird is stuck to one device (desktop). Technically you can have Thunderbird across a lot of computers by using IMAP instead of POP3 but that slows down and cancels out a lot of your speed benefits.

=== Why Mozilla Should Fix Them ===

- Originally I had typed up a HUGE list of things that desktop email clients can do that web mail clients cannot. Upon further inspection I found that a LOT of those features, everything from multiple accounts being displayed in one stream and searching across multiple accounts is now available in gmail.

- Email Clients allow me to have full control over my email inboxes and contacts without having to feed them into gmail.

- Email Clients give me a lot of options in how I can display, index, read, and write email.

- Email Clients allow me to search emails and contacts from across ALL my email accounts (gmail currently has a limit of 5 accounts).

- Email Clients allow you instant one click access to all your email accounts with powerful and expandable features, an intuitive and lightning fast interface, and god-like control over massive amounts of email accounts. For business people, entrepreneurs, assistants, community organizers, and domain owners email clients are a necessity.

- The same way power-users like using Seesmic for twitter and facebook, and people like downloading and using native apps over web based ones, the speed and control of software is what's keeping me with Email Clients at the moment.

- As soon as you have more than 5 email accounts to manage on a daily basis, the speed of an Email Client wins out. Gmail only allows 5 multiple accounts to be imported into your stream.

=== Why Mozilla Will NOT Fix Them And Instead Leave Thunderbird ===

- Everything I mentioned above is slowly getting emulated by web mail. At the moment gmail is the winner when it comes to email client emulation but in a few years I can see an elegant php+mysql web based email client that not only does exactly what Thunderbird does, but does it across all your devices. And without breaking all your addons after every update.

TLDR: The end is near for Outlook + Thunderbird + Mail + Evolution + The Others...