A bit late to the party, but posting for reference in case someone finds this post in the future. For Ubuntu, I did as the following:

1. Created the chrome folder at `~/.mozilla/<your-profile-id>.default/`

2. Created `~/.mozilla/<your-profile-id>.default/chrome/userChrome.css` and added

    @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

at the top of the file.

3. Added customizations after this line. The resulting file will be like:

    @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 
    /*
     * TreeStyleTabs solutions to remove tabs
     *
     * Solution by: https://news.ycombinator.com/item?id=15343940 */
    #TabsToolbar, #sidebar-header {
      visibility: collapse !important;
    }

    #TabsToolbar {
      margin-bottom: -21px !important;
    }

You create it in the "chrome" folder in your Firefox profile. The "chrome" folder itself may need to be created.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-... has directions for finding where the profile lives.

C:\Users\wakkaflokka\AppData\Roaming\Mozilla\Firefox\Profiles\random_gibberish_identifier\chrome\userChrome.css