How to bypass startup window asking me if I want to make changes to my device

That sounds like the Windows User Account Control prompt that appears when Firefox starts or updates. You generally shouldn’t bypass it by disabling UAC entirely. If the prompt appears every time Firefox launches, I’d check Firefox’s shortcut or startup entry to make sure it isn’t set to run as administrator. Right-click the Firefox shortcut → Properties → Compatibility and make sure “Run this program as an administrator” isn’t enabled. Also check the shortcut’s Advanced settings for the same option. If it started after a Firefox update, a clean reinstall from Mozilla’s official site may also fix a broken updater or shortcut.