Please consider turning on send info to Mozilla before posting a question. This helps us help you.

Should be easy, I hope. Start Menu then look for the Start Up Folder listed there and open and delete Firefox with a right click

Note : Firefox 57 Quantum release date Nov 14th https://www.mozilla.org/en-US/firefox/quantum/ Note : Legacy Extensions will be disabled and or removed.

Please let us know if this solved your issue or if need further assistance.

That's if Firefox opens normally with its homepage. If something else is displayed, tell us what it is.