Did you move the French version from the Application folder to the trash?

You can download the English version from this page:

• Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html

Yes, then I followed instructions and found a library file that contained preferences and trashed that too. I downloaded the English version from the page you suggested and although I am able to get the help and other pages in English (I made them the homepage until I become familiar with the system) the headers are still in French, and the previous homepage I'd chosen still came up until I changed it.

Which headers are still in French?

The menu bar and other items in the user interface should be in English.
It is possible that bookmarks folders and default bookmarks and the Google home page are still in French.

The default bookmarks and Google homepage are still in French. I haven't personalized the site yet because I wanted to do it on the English version. It seems to be part in French and part in English, which is OK because I understand both, but I would prefer to either delete the French version from my system or to have 2 separate programs. Thanks for helping...

If the default items on the Bookmarks Toolbar or in the sidebar are in the wrong language then you can rename them to the English names (right-click: Properties).

Bookmarks menu: Bookmarks Toolbar, Recently Bookmarked, Recent Tags, Get Bookmark Add-ons
Bookmarks Toolbar: "Most Visited", "Getting Started", "Latest Headlines"
Main folders: "All Bookmarks", "Bookmarks Toolbar", "Bookmarks Menu", "Unsorted Bookmarks"

Yes, that worked. Thank you so much!

You're welcome