How can I start Firefox with add-ons installed?

Create a Firefox profile and add required add-ons. 

Take ‘Firebug’ as an example, save the Firebug XPI into the C:FF_addons folder as firebug.xpi (Download XPI file from Firebug download page, right-click on the “Download Now” and save as C:FF_addonsfirebug.xpi).

Then create a Firefox profile and add Firebug add-on while instantiating FirefoxDriver as below:

   String firebugFilePath = "C:\FF_addons\firebug.xpi";
   FirefoxProfile profile = new FirefoxProfile();       
   profile.addExtension(new File(firebugFilePath));
   // Add more FF addons if required 
   WebDriver driver = new FirefoxDriver(profile);