Unfortunately I can not remember the password of FTP server, but it is saved in FileZilla Site Manager (Ubuntu).
How can I view the saved password?
Export your Site Manager information and you'll find them in plain text:
File > Export > Export Site Manager entries
It exports everything in XML like this:
<Server>
<Host>ftp.example.com</Host>
<Port>21</Port>
<Protocol>0</Protocol>
<Type>0</Type>
<User>root</User>
<Pass>mypass</Pass>
[etc]
</Server>