Back in 2014, when knighty removed the banners, I took the userscript by PiercingSight and modified it in a way so that not only the banners are shown, but the old site logo is on them as well. You can find this userscript here:

https://userstyles.org/styles/108589/return-banners-and-the-old-logo-to-fimfiction-net

With the recent site downgrade, this userscript didn’t work anymore, but with the help of another userscript, the one by alamais that he made to put the banners behind the user bar, I was able to figure out which lines I have to replace.
The userscript does now show the banners again between the black and the Microsoft Blue Green nav bar. You can switch out the banners by replacing the URL as detailed in the notes of the script.
Unfortunately, the logo does still not work. Including the second URL for the logo leads to the script becoming defunct immediately.
I want to fix this by directly copying the logo on the banners, saving them under a different name, then uploading them myself and replacing the links to make the style work again with the logo.
It would be a lot of work, though, so this isn’t going to happen anytime soon. Feel free to take a look at the code and tell me what I need to change to make the logo work again, if you know how to fix this problem.
I will apply the necessary changes and bring it back then.

The logo is back now! Thanks to Sollace and once again alamais' userscript, it is back where it should be!^^ Details here:

https://www.fimfiction.net/blog/742221/userscript-update-2-return-the-banners-with-the-logo-d

5981334
Dood.

@-moz-document regexp('http.*fimfiction\\.net.*') {
  html:not([FimFic_Adv]) .user_toolbar:before {
    display: block;
    height: 175px;
    max-width: 1300px;
    content: '';
    background: url('https://static.fimfiction.net/images/logo-2x.png') top 50% left 10% / 200px auto no-repeat,
      url('https://static.fimfiction.net/images/custom_banners/solar_group.jpg?') center center;
    margin-left: auto !important;
    margin-right: auto !important;
  }
}

5981334
Sollace already brought the banners back in their FIMFiction Advanced userscript, though.

5981367

Thanks! Is there a way to display the logo smaller? Currently, it takes up most of the banner if I include it, because the size of the uploaded logo is really big.

5981420
Yeah, I adjusted the code. It'll do you well to look up a bit on how css works.

5981371
A little variation doesn't hurt.

5981725

Thanks. I should do this, yeah, but I don't really have time for this. Creating this userscript two and a half years ago was a completely spontaneous move, because I wanted not just the banner, but the old logo back as well. I'm not normally creating any userscripts and won't create any more than this one.
Between my writing and several other things, I simply lack the time to learn coding.

I updated the userscript again and the logo is back too now! Details are linked in the start post!