Did Firefox mention any problems during the update, such as needing to create a new profile or perform a tune-up?

More generally, it sounds like your website cookies were cleared, because if Firefox doesn't send those cookies, the sites can't verify that you were signed in. It's difficult to understand why your Instagram account status would have changed. Assuming you see the same problem when you check the site in Microsoft Edge, can you contact their support about what is going on?