Turning off sync does not remove any data from the user profiles. If you want to remove something, you need to do it yourself.

Why would my user profile information still be available in the browser even after logging out? What is the point of having "secure lock" or whatever it is called if anyone can just see all my passwords even if I am not logged in? Why is all the password info saved even after clearing cache and browser data?

Only someone with access to your computer user account will have access to your Firefox Profile.
In the browser, the passwords are encrypted using two files.

The purpose of Sync is to share local data among your different installations of Firefox. After the data is shared, disconnecting from Sync does not delete it because it was never intended to be temporary.

To protect against third party access to your saved logins, you can set a local Master Password. See: Use a Primary Password to protect stored logins and passwords.

Alright, I was under the assumption that when you have a firefox account and you log in to a browser, that is what gives you access to all your information including usernames/passwords. Seems kind of odd that no matter what I do: logout, clear cache/data, that my information is still in the browser, meaning there is no way to remove your usernames and passwords from a Firefox browser. It is also odd that when I log in to my Firefox account from a new computer that all of the logins automatically show up, but the reverse is not true. I just want to make sure that when I leave my office that no one else can log in to all of my accounts, including my Firefox account.

I still want to know how to delete all stored information in the Firefox browser, especially when I log out of my Firefox account and also clear cache/data. How do I make sure my login info is completely removed?

You can use the free app CCleaner to clear all your firefox data by one click. CCleaner > Custom Clean > Applications > Firefox > tick the wanted gaps

There is a difference between how Lockwise on desktop handles logins and the Lockwise app on mobile devices. On desktop Firefox stores the logins in logins.json in the profile folder, so they are available locally even if you aren't connected to a Sync account. If you use Sync the your logins are uploaded to the Sync server and the Lockwise app on mobile can retrieve the logins from the Sync server and display them. I.e. Lockwise on mobile doesn't store the logins on the device, but always loads them from the Sync server.

Seems kind of odd that no matter what I do: logout, clear cache/data, that my information is still in the browser, meaning there is no way to remove your usernames and passwords from a Firefox browser.

These are all individual buckets of data. I think 99% of users would be infuriated if clearing history removed their saved logins. But you can clean them up here:

menu button > Logins and Passwords

In Firefox 76+, there's a new authentication requirement: Firefox asks for authentication (password, voice, face or fingerprint) when accessing passwords.

I still want to know how to delete all stored information in the Firefox browser

With this, you can put it in your pocket.

http://portableapps.com/apps/internet/firefox_portable Mozilla Firefox, Portable Edition

https://portableapps.com/apps/internet/firefox-portable-esr Mozilla Firefox ESR, Portable Edition

A fully functional package of Firefox optimized for use on a USB key drive. A specialized launcher will allow most favorite extensions to work as you switch computers.

Firefox Portable is a 3rd-party build. Support is available here: http://portableapps.com/forums/support/firefox_portable

You can use sync to share data between profiles. But some data cannot be shared between different versions.