I could live with Thunderbird's unflat interface. Its very difficult to configure. If TB folks could use some of Microsoft's email programs for a while, they could vastly improve their program. TB is fast, VERY fast when compared to MS offerings. Multiple email, nntp and RSS feeds make it very appealing.
But when it drops ALL your passwords for ALL accounts and REFUSES to accept them, then the program is useless. I can't access any newsgroups or any email. When configuring newsgroups and email accounts, there is NO configuration for setting up your accounts like there is with Windows Live Mail or even Mailwasher (a nice anti-spam program). Where does one set up passwords? There is no configuration for passwords. You have to run the program, which then prompts you for the passwords and an option to save it somewhere.
But when Thunderbird loses the passwords (and it will), there is no recovery. You are hosed. Shame really, as TB has promise, but the developers make it so damn hard to use and configure.
Help is only available via the browser and no help or PDF files. So if you have no internet access, you are buggered.
STAY AWAY! It's open source which means improvements and new features are very slow in coming. I've been watching TB for years waiting for it to mature enough for me to use.
This is not an acceptable program for use. Sorry — but it needs a LOT of work before its usable.