I had the same problem and I think I have the SOLUTION. Spent 2 whole days tearing my hair out trying to fix it. Followed all the advice…de-activated all my plugins, created PHP.INI and set the memory higher:
;; set memory limit for cache.php
memory_limit = 64M
Did the same with wp-config.php and so on. Nothing worked so I downgraded Firefox and downgraded WP. Still nothing.
I ripped out WP and reinstalled. Still nothing.
Finally, I started installing my plugins. The first one was FS Contact Form, and as soon as I did I saw an error messages saying, “Warning: Your web host has PHP safe_mode turned on. PHP safe_mode can cause problems like sending mail failures and file permission errors. Contact your web host for support.”
I called, asked them to turn safe mode off. Refreshed my editor page and BINGO! The visual editor WORKED!
Yaaaaaaay!
Hope this helps others.
Cheers
Mike