FWIW I do the same. It's the first thing I do on new installs of F'fox or Waterfox:
1. Open my email, download my `userchrome.css` file
2. Go to Help | More troubleshooting info | Open my profile folder
3. Make a folder called "chrome" in it.
4. Open my downloads folder.
5. Move the .css file from "Downloads" to "chrome".
6. In F'fox: go to `about.config` and enable legacy stylesheets.
Yes, tweaks going out of date was also my concern, but they're already a few years old and nothing broke so far. I reckon I could just adapt them if anything breaks, they're easy.
[0] https://mrotherguy.github.io/firefox-csshacks/