Are the passwords being controlled by a password manger that is part of some security suites, like Avast, or an app like KeePass?

They are not. I'm manually managing my passwords.

In most cases, password-saving issue are fixed by deleting key4.db, cert9.db, logins.json and pkcs11.txt, while TB is closed. If that doesn't work, you could try creating a new profile from Help/More Troubleshooting, about:profiles, add your account, and see if the passwords work properly. Make sure you use the correct settings in case your current authentication settings are obsolete:

https://support.mozilla.org/en-US/questions/1354779

When I create a new profile, it adds my server credentials to the saved logins list and remembers my password.

Am I able to transfer files to/from this new profile to/from the defaultuser profile so that I can retain my local emails, preferences, add-ons?

Local mail can be transferred from the old profile by copying the mbox files - the large files with no extension, named after folders - in Mail/<popserver> and Mail/Local Folders, and .sbd subdirectories, to Mail/Local Folders in the new profile.

If the address books were properly updated to the sqlite format in the old profile, copy abook.sqlite and history.sqlite to the new profile. Preferences can't be copied, but you can launch the old profile alongside the new one and compare settings. Install the latest versions of add-ons that work in 91.

How would I go about opening two copies of Thunderbird with separate profiles on them? I was able to do so when creating the new profile, but now I'm having trouble finding the option to open a copy normally. This version of Thunderbird doesn't allow me to simply open another instance like the previous one did.

My contacts list must not have updated on the old profile because transferring it to the new one didn't work. I was able to get my local emails into the new account.

Also now my issue is it once again redownloaded all my mail on the server and now re-adding the "Remove Duplicate Emails" add-on, it does not detect duplicate emails despite the fact that I'm looking right at them. This was my whole reason for updating Thunderbird and now it's invalidated. It worked fine on the account that wouldn't save my password, even using the same filter options.

Edit: I was able to get the plugin working. Had a corrupt install. While trying to play with the various individual filters, all the related windows & tabs closed and the plugin just disappeared from my addons. Had to delete all the remnant files from the local profile folders, re-add the plugin, and it worked afterwards.

If TB is opened with a profile, another profile can be launched from about:profiles.

If you have any .mab files in the old profile or .mab.bak, such as abook.mab.bak, remove the .bak and import it from Tools/Import...Address Books...Mork database... select a .mab file.

Check that you have v. 0.5.2b4 of the Duplicates add-on:

https://addons.thunderbird.net/en-US/thunderbird/addon/removedupes/

Excellent. I have my contacts back in, plugins are working, old emails are back, and the password saves. New Thunderbird is roughly 4 times slower than old Thunderbird, but when I update my system, it will probably speed up.

Now I assume I can remove my old default profile (since this new one appears to be current default) through about:profiles and save some space on my SSD?

If TB is significantly slower, check that the profile folder is added as an Exclusion or Exception in antivirus settings, or disable Mail Shields etc.

Yes, profiles can be deleted from about:profiles.

Last question, but this one might be a long-shot. Was supposed to get 2gb of storage back after deleting the corrupt original profile. I removed it through about:profiles which warned me it would be deleted, but I didn't get any space back afterwards. The folders weren't in my recycle bin; do you know if removing it through about:profiles drops it elsewhere on the installed drive? Or perhaps it won't be reflected until restarted.

I don't know where the deleted profile is stored, or if it is stored at all, but a restart of Windows might clear it.