do I always need to sign in?

Into Firefox? No you can use it without signing in. However, if you want to sync your account you would have to.

Hi Crispen, Firefox recently had a bug where you had to enter a Primary Password at startup even if you weren't connected to Firefox Sync or trying to sign in to a site using a login saved in Firefox's password manager. That should be fixed in Firefox 136.0.2. See: https://www.mozilla.org/firefox/136.0.2/releasenotes/

If you meant something else, could you explain in more detail?