Firefox lets you import settings and data from Opera. This can be handy when you start using Firefox because it saves you from having to configure many things yourself.

The first time you run Firefox, it will automatically ask you if you want to import data. If you chose not to or if you wish to do it again, you can start the process again.

1. From the Firefox menu bar, click the File menu, and select Import....
   
   
   
   
2. Firefox can import settings and information from many other browsers. It will list browsers on your system that it can import from. Select Opera, then click Next.
   
   
3. Firefox will list the types of settings and information it can import. Generally, you will want to import everything. Select the items you want to import, then click Next.
   
   
   • Preferences: Various settings, including your home page. Since Firefox and Opera have different features, Firefox won't be able to import all of your settings.
   • Cookies: Small files stored on your computer by some web sites that are used to keep you logged in, store your options, or do other things.
   • Browsing History: Information on the sites you have visited.
   • Bookmarks: Web pages you have saved in your Opera bookmarks.
4. Click Finish. The items you selected should now be imported.
   

This article originally derived from Import bookmarks (mozillaZine KB)