THIS IS NOT YOUR TYPICAL GUIDE, IF YOU DON'T READ THIS POST AND ONLY COPY THE CSS, IT PROBABLY WON'T WORK OUT FOR YOU

Okay, since several people are asking for the CSS, so I want to preface by saying I am at best a script kiddie that has stolen 99% of this from here and there around the net. I cannot properly give credit where it is most certainly due for the original authors of any of this, and it IS a big mess I've thrown together over time. That being said, it may not function perfectly for you out of the box, due to variables changed in my about:config, addons I have installed, etc. Hopefully it just works for you, but no promises.

First up is the userChrome.css file found in your profile folder and then the chrome inside that, mine has a bit at the beginning for the addon SimpleMenuWizard, which you can remove, but I included it for full transparency in what I'm working with here:

https://pastebin.com/C6STwwmb

Next up, I have the addon TabCenterReborn which was used for the tabs on the side, you absolutely can accomplish this with only CSS if you know how. I do not know how. So I'm using that addon and it has it's own CSS page in it's settings. Here is what I have there, but warning: it's an even bigger mess and I am sorry:

https://pastebin.com/tLg1jmVp

I also use the following addon and theme that may or may not be relevant to you in getting the Firefox you want:

Custom Scrollbars (for the thin scrollbar, again can probably just be done with CSS, but I am dumb)

The Darkspace theme for the darkest theme with still just a little bit of flair I could find.

For shits and gigs if you are interested I will also list the rest of the addons I use if you want to check them out/if you have recommendations for a better alternative:

Dark Reader (never found a perfect solution for every site out there, but this one works pretty well for getting rid of really bright sites killing your eyes)

DeArrow (community project to replace all clickbait titles and thumbnails from Youtube with appropriate ones, it's probably better than I'm making it sound)

Download Manager (S3) (I just personally really like this addon for managing my downloads, highly personal choice here)

Image Max URL (helps you find the largest resolution version of an image, can automatically replace them in websites like it wasn't ever there to begin with)

Nightly Tester Tools (I'm not so sure how important this one is these days, kept it out of habit, you probably won't need it?)

NoScript (you don't already have this? why?)

Reddit Enhancement Suite (tons of options of configure Reddit to your liking, but these days, who even wants to go to that Dumpster fire)

Rotate and Zoom Image (you can add rotation and zoom options to your context menu when you right click, might be useless for many people)

Search by Image (this is essentially just a shortcut in your context menu for copying a link to Google image search, like others, there might be a more elegant solution to this than using an addon, but it works for me)

Tab Session Manager (saved my mental sanity several times, lets you manage sessions to a pretty good degree right in your browser)

Tampermonkey (there are a few options for running custom scripts on sites, I personally use Tampermonkey, but I can't remember why I preferred it to any others, you'll have to look into this one to fully understand all the potential this unlocks for you)

uBlock Origin (see comment for NoScript, make sure you enable all the extra blocking stuff in the settings (make sure you aren't using Adblock Plus ever))

VideoDownloadHelper (pretty self explanatory, helps you download videos from sites that don't make it as simple as it should be to do so)

Hopefully that is enough information to get you guys who want similar close enough. If anyone reading this has suggestions for improvements on what I have set up here, I'm certainly open to hearing them, and I'm MORE than welcome to anyone trying to clean up my mess of CSS.

EDIT: Already finding flaws in this I need to figure out, lol. The tab bar for me is not appearing by default in private windows, it is appearing by default in popups like Google logins and downloads, and it's not accessible at all when in fullscreen for me when on the edge of my browser that goes into where my two monitors meet.