Trouver le profile Firefox :

• Linux : ~/.mozilla/
• Windows: "%APPDATA%\Mozilla\"

Dans le dossier du profil, créer un dossier chrome, puis un fichier userChrome.css à l'intérieur de celui ci, contenant :

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

#TabsToolbar {
  visibility:collapse;
}
#sidebar-header {
  display:none;
}

EDIT:

You need to set toolkit.legacyUserProfileCustomizations.stylesheets to true with about:config, otherwise userChrome.css (and userContent.css) in your profile is simply ignored by Firefox.

EDIT2: Upated for windows Sidebery:

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

#TabsToolbar .toolbar-items #tabbrowser-tabs,
#TabsToolbar .toolbar-items .toolbarbutton-1 {
  display:none;
}
#sidebar-header {
  display: none;
}

EDIT3: In latest Ubuntu symlink are not working because Firefox is a snap package. I had to create a regular file in ~/snap/firefox/common/.mozilla/firefox/