I tried it and it does indeed work, however, there is one small change to make.

When you change Inbox.mbox to just InBox in Finder, it becomes(always was) just another folder, which contains a file called mbox this is the file which you want to move into the specified thunderbird directory and rename to match whatever folder you are replacing.

Also in the thunderbird folder in addition to the InBox file, there is a Inbox.msf file, if you have ever had anything in the inbox. It is probably a good idea to remove that file as it seems to be an index of the actual InBox fle. Might get a bit messy if you dont remove it. Though I didnt decide to test it.

I am just trying Thuderbird out to see if I like it, so I can't yet comment on it compared to Mail.app, however, compared to Entourage it is great. Of course I switched from mail.app to Entourage and then switched back to mail.app after deciding that Entourage sucked. I specifically hated the search function in Entourage, it was fairly sophisticated in that you could refine your searches very well, but, it took for freaking ever to run a search on my mailboxes. Yes I did have a very large Mailbox at the time, but Mail.app was able to search it quite quickly.

So far I have only one complaint about Thunderbird, if you have multiple accounts, it will only let you search in one of them at a time. I worked arround that by setting filters so that all the mail goes into local folders anyway, so it wasnt a big deal.

I am happy to report that the reverse process, moving mail back to Mail.app works great as well.

To import, open mail.app, create a mailbox, close mail.app.
go into that mailbox in the command line, or rename it in finder getting rid of the .mbox extension so you can go into it

/Users/username/Library/Mail/Mailboxes/mailboxname.mbox/

delete content_index and table_of_contents, then copy the Thunderbird file which contains the mail you are moving into this directory, rename it to

mbox

then if you changed the Mailboxname.mbox folder in finder so you can get into it, change it back to Mailboxname.mbox

open mail.app and it will re-create the index and table of contents.

I like Thunderbird, I really did, but the lack of support for vCards, no builtin Calendar app, yeah I know Mail.app doesnt have one, but it can be used with iCal. Basic integration with other parts of the OS was a problem, for example integration with a browser, Thunderbird is a stand alone email app, which is fine, but there was not any easy way to use it in conjunction with Safari(i.e. click a mailto link) or even Mozilla 1.6 or firefox.

Since I was not on a crusade for opensource, I switched back.

Mail is very good, particularly dynamic searching across multiple mailboxes. My big problem was lack of "read receipts". Most of my mail recipients don't send acknowledgements but won't block read receipts, so after wasting a lot of time chasing responses, I haved moved reluctantly from Mail to Thunderbird.