Version 16 of thunderbird works on mac 10.5
Though.. You may need to reset the password ***** the mac to install it, since you said you dont know the password ***** when the mac starts to sign in ( which is the same password ***** when you install programs )
This link will walk you through resetting the password ***** the user account in question, but booting into single user mode
Keep in mind, in the last line where it says /Users/username password
You must replace "username" with the actual user account name you want to reset the password ***** And replace the word 'password" with the password ***** want to use.
For instance, lets say you were trying to reset the password ***** an account / user called JohnsMac and you wanted the password ***** be July2016
You would type in dscl . -passwd /Users/JohnsMac July2016
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