Is it possible to easily retrieve Thunderbird's passwords with access to HDD?

On linux, the password database is stored in:

/home/$USER/.thunderbird/$RANDOM_STRING.default/signons.sqlite

See @Karrax's answer for Windows locations.

You can examine this file interactively using the sqlite3 CLI:

sqlite3 ~/.thunderbird/zxcv1357.default/signons.sqlite

sqlite> .tables
moz_disabledHosts  moz_logins
sqlite> .schema moz_logins
CREATE TABLE moz_logins (id                 INTEGER PRIMARY KEY,hostname           TEXT NOT NULL,httpRealm          TEXT,formSubmitURL      TEXT,usernameField      TEXT NOT NULL,passwordField      TEXT NOT NULL,encryptedUsername  TEXT NOT NULL,encryptedPassword  TEXT NOT NULL,guid               TEXT,encType            INTEGER, timeCreated INTEGER, timeLastUsed INTEGER, timePasswordChanged INTEGER, timesUsed INTEGER);
CREATE INDEX moz_logins_encType_index ON moz_logins(encType);
CREATE INDEX moz_logins_guid_index ON moz_logins(guid);
CREATE INDEX moz_logins_hostname_formSubmitURL_index ON moz_logins(hostname, formSubmitURL);
CREATE INDEX moz_logins_hostname_httpRealm_index ON moz_logins(hostname, httpRealm);
CREATE INDEX moz_logins_hostname_index ON moz_logins(hostname);
sqlite> select * from moz_logins;
3|imap://imap.example.com|imap://imap.example.com||||MEIEEPgAAAAAAAAAAAAAAAAAAAEwFAYIQwErTyUiOp12345GmuM2KNXcZ=|MEIEEPgAAAAAAAAAAAAAAAAAAAEwFAYIQwErTyUiOp12345GmuM2KNXcZ=|{1234abcd-beef-feed-face-0a0a0a0a0a}|1|1320123123123|1320123123123|1320123123123|1
4|smtp://smtp.example.com|smtp://smtp.example.com||||MEIEEPgAAAAAAAAAAAAAAAAAAAEwFAYIQwErTyUiOp12345GmuM2KNXcZ=|MEIEEPgAAAAAAAAAAAAAAAAAAAEwFAYIQwErTyUiOp12345GmuM2KNXcZ=|{1decafbad-fa11-1234-1234-abcdef0123456}|1|1320123123123|1320123123123|1320123123123|1

If you wanted to fetch usernames/passwords from code, it's as simple as:

echo "select encryptedUsername, encryptedPassword from moz_logins;" | sqlite3 ~/.thunderbird/*.default/signons.sqlite

or the equivalent in your favorite programming language with sqlite3 bindings.

Of course, if they're encrypted (as shown above) you'll need to put some effort into guessing the master password used for encryption. As a user, know that if you use a weak master password (e.g. P4ssw0rd1) it will be trivial to get the cleartext passwords.

In Thunderbird 8.0, I can easily see all my passwords in the Options window, in Security tab, in the Passwords tab, in the Saved passwords window, with the "Show passwords" button.

I am not sure if you meant "how do I access the passwords programmatically".

The answer is yes.

ThunderBird stores all remembered email settings along with password into the SQLite database file 'signons.sqlite' in its profile location. The default profile location for different platforms is as follows,

[Windows XP]
C:\Documents and Settings\<user_name>\Application Data\Thunderbird\Profiles\<random_name>.default

[Windows Vista & Windows 7]
C:\Users\<user_name>\AppData\Roaming\Thunderbird\Profiles\<random_name>.default