Since the Dark Theme in Firefox 68.0.2 still has a flashing white page when opening a new tab, it’s no useable dark theme in my opinion. 
That’s why I prefer ShadowFox (Github) instead of the native dark theme. Easy to install and many ways to do further customizing.
Firefox Add-ons I use:
- Bitwarden
- CanvasBlocker
- ClearURLs
- Cookie AutoDelete
- Dark Reader
- Decentralyes→ replaced with LocalCDN
- Firefox Multi-Account Containers
- Facebook Container
- Google Container
- First Party Isolation
- Flagfox
- MyJDownloader Brower Extension
- OneTab
- SoundCloud MP3 Downloader
- Startpage.com — Private Search Engine
- Swift Selection Search
- Tree Style Tab- Plus this colored theme CSS
 
- uBlock Origin
- I don’t care about cookies→ instead activate the uBlock and Adguard Annoyances Lists in uBlock or try Consent-O-Matic
- Switch for PiHole
- Wallabagger
- Keepa
- User-Agent Switcher and Manager
- LASER CAT( ͡° ͜ʖ ͡°)
- Update Scanner
- YouTube for TV
- Enhancer for YouTube
- SponsorBlock for YouTube
- Feed Preview
- Image Max URL
- Textarea Cache
- Grammatik
- Tampermonkey
- Chrome Mask
Also, I like bookmarks in the toolbar centralized. Just append the following lines to the userChrome.css.
The file is placed in your Firefox Profile: ~/.mozilla/firefox/profile_name/chrome/userChrome.css
#PersonalToolbar {
  display: flex !important;
  justify-content: center !important;
}
