This is a post about a WordPress problem and its already-figured-out solution that I'm posting so others don't have to refigure this out.
I installed WAMP to run WordPress on a local machine so I could develop an already-live site without disrupting traffic. I downloaded the MySQL database from the live site (which is running 3.4.2 but has been updated from previous versions of WordPress multiple times), imported the MySQL database into my local databases through phpMyAdmin, and tweaked all of the database tags that refer to the site URL so that they would point to the local machine directories.
After getting WAMP and WordPress installed and running (and solving another problem in which permalinks were not forwarding, which required enabling "rewrite_module" under WAMP's Apache Modules), I had the aggravating problem of everything working fine EXCEPT the visual post/page editor not appearing in the admin backend. What was more irritating was that this was only happening in Firefox (version 15.0.1), but everything was fine in Internet Explorer.
After sorting through my Apache error log, I found that at some errors had been generated complaining that a tinymce file could not be found:
File does not exist: C:/wamp/www/sitename/wordpress/wp-includes/js/tinymce/langs/en.js, referer: http://localhost/sitename/wordpress/wp-admin/post.php?post=100&action=edit
So I went and checked out what was in the /wp-includes/js/tinymce/langs directory, and sure enough there was no en.js file, but there was a wp-langs-en.js file... so I made a copy of this file and renamed it en.js and all of the sudden everything worked fine.
Possibly this is an artifact of the existing MySQL database counting on files that existed in previous installations on the server, but what's weird is that there is no "en.js" file on the server for the live version of the site, whose visual editor works fine in Firefox, just a "wp-langs-en.js" file.
In conclusion, the problem of the visual editor not displaying in Firefox only on a WAMP installation of WordPress was solved by making sure there was a second copy of the "wp-langs-en.js" file in the /wp-includes/js/tinymce/langs directory named "en.js".