Tour of ~/.mozilla

• Profile directories
• "Salt": profilename/gibberish.slt
• bookmarks.html
  
• Cache
• prefs.js vs. user.js: the former will be overwritten by the app, the latter is safe so put any unusual prefs in user.js.
  
• Mail and ImapMail, plus various mail files like training.dat
  
• chrome, userChrome.css, and userContent.css: Use these to override defaults for the look of the browser window (userChrome.css) or content shown in the browser's content area (userContent.css).
  
• history.dat, localstore.rdf, various other RDF files
  
• plugins: If you put plugins here, you won't lose them when you upgrade to a new version of Mozilla.