S-Mode is an appliance mode for Windows that prevents installation of any app not from the Microsoft Store. Disable S-Mode and you can install any application like you’ve always done, including Firefox.

Fedora should play nice with Secure Boot enabled. You also shouldn’t need to do anything with the TPM.

Depending on the model and brand, there’s probably a YouTube video showing how to disassemble it. This can help you find the keyboard connector and how to disconnect it.