You must install Firefox before installing Welsh.
You must install the Locale Switcher as well as Welsh.
You are about to download a Firefox language add-on. Add-ons are small pieces of software that add functionality to Firefox. In this case they will re-create your existing version of Firefox in the language of this add-on.
Upon installation you will need to re-start Firefox, after which it will appear in the new language. You can switch back to the original language, using the language switcher which is installed with this add-on. This menu option will appear within the "tools" menu, titled "languages".
All of these language add-ons are built by open source developers and are in early stages of review, so there may be errors in the translation. If you find translation errors, and would like to help us improve this add-on, you can do so by posting your comments. That way the person who built this add-on will get your feedback and can make the necessary fixes.