Please share more info? Windows? what release of thunderbird? do you have any email accounts working? Is your addressbook accessible? You mention having messages on D drive. Are those messages within an account, or were they previously under Local Folders? My guess is that, in reinstalling, you may have created a new profile. Let me know.

re :I Updated to Your Latest Thunderbird (Default) Update: And Now I Can't Open Addresses in A-Separate Window....

You need to tell us why this is not possible. Are you using a 'double click' action ? Is this set up:

• Settings > General
• scroll to 'REading & display'
• Open messages in:
• Select 'A new message window' or 'an existing message window'

This means a double click would enact that option.

OR are you right clicking on selected email and choosing 'Open' and then 'Open message in new window' . What options do you see when you select 'Open' ? Upload an image file eg: jpg showing what you see.

re :So I went Back to Previous Installation:

You cannot simply reinstall a previous version and expect it work. Ever since version 68, https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation

This is what you can do if you want to use that earlier version

• Exit Thunderbird.
• Access profile name folder and delete the 'compatibility.ini' file.
• Start Thunderbird
• select the correct profile.

If you need additional steps please ask and they can offered.

Please note if you have auto updates operating then remember to update those settings to select tell you about apdate but let you choose whether to install.