You will need to install a user script manager extension to install this script.
As the name says, this script removes the annoying login requests from Instagram, Facebook, YouTube (mobile only, for the desktop version, try Youtube - dismiss sign in), Reddit, Twitter, Quora, Ask.fm, VK and Pinterest.
Quora: if you want to login while the script is enabled, just go to the main page (quora.com) and login from there.
Youtube: this script works only for mobile Youtube interface; for the desktop version, try Youtube - dismiss sign in
Instagram: If you try to open an Instagram profile page (es: instagram.com/mozilla), and you get redirected to the login page (instagram.com/accounts/login/), this script can't do anything, as this means that your IP has been blocked by Instagram, unless you sign in. You can try a different connection or you can try to refresh your IP from the modem/router's settings. If this fails, you can still use https://imginn.com, a site that can show any public Instagram profile without logging in and without unnecessary IP restrictions.
Facebook Marketplace: it requires the users to be registered to access. It's not just begging you to login, it's forcing you. No login no access: this script can't do anything for it.
Instagram: they recently introduced an annoying login wall that replaces the third image in a carousel with a login request (only on mobile interface). I currently have no idea, nor will, to find a workaround for it, but if you need to see that blocked image from your phone, just set your browser to Desktop Mode and you wull see the picture as usual.