The topic about email clients is locked so here's a couple of tips on using Thunderbird. It can be left with a simple setup or configured for power users.
1) Development on Thunderbird has not stopped. The Mozilla project is only focusing on stability with innovation pushed to the community. v17 was just released but obviously email technology is not changing as fast as other things so I would expect changes to be more incremental than revolutionary anyway.
2) If you want to set up multiple profiles I've used a couple of options.
Tbird does a great job of organizing folders under multiple email accounts but you can also use the built-in profilemanager. Either set up a shortcut or use WIN+R and then type this:
If you clear the default check in the "Don't ask at startup" then the profile selector will come up everytime you start Thunderbird (assuming you have more than one profile)
3) You can get to advanced settings (about:config). Just use Tools->Options->Advanced->GeneralTab->Config Editor. You can set a lot of defaults like fonts and such. Just remember that hacking on Thunderbird's config can get you into a bad spot faster than with Firefox.
4) Consider using IMAP rather than POP3. IMAP is slower and does tend to use more space on the server but it does more of a "directory sync" than a download. Even with POP3 leave mail on server option (barely useful when not broken), IMAP will do a much better job of syncing email across multiple devices and that "full sync" can come in handy if you ever have a HD crash.
5) Unless you are in an Enterprise/Corporate environment Microsoft email clients are bound to give you a headache sooner or later. Aspirin is still cheap if you would rather fight than switch.