Hi Jeremy, you can create a new profile to start Firefox, and then investigate further:

How to run Firefox when your profile is missing or inaccessible

But first, if you haven't restarted Windows since this problem began, please try that as this sometimes is caused by locked files or incomplete permissions.

This morning it did it again. I was kind of frustrated, so I went into the App Data folder and remembered that I had accidentally moved it to my OneDrive. It worked perfectly fine in my OneDrive, until the Microsoft 365 admin made me change my password. That's when it went all wonky.

Now it's up and running again.