When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Offline Storage : "Clear Now"

2. If needed, delete the site's cookies here

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?

Is it a regular HTTP uploader or does it use Flash?

That didn't seem to help. (I tried number 2, logged-out and refreshed the page).

It doesn't use Flash. It is a text editor used by Joomla cms. I click browse, locate the jpg on my desktop and then, I get the file unrecognized error.

The reason it must be FF is that this function works when I try it in Safari. I guess it might be the JCE editor that has bugs with FF 12. :(

Can I go back to an earlier version of FF?

Do you have the ability to update JCE? This thread suggests it could solve the problem: Joomla! - View topic - Problem in uploading photos in JCE editor. Another one suggested disabling a different uploading option (but this means nothing to me): File type not supported - File: undefined | Image manager extended 2.0.2.

Wow! I posted the problem in the Joomla forum, and tried to search for the answer beforehand. You found the exact solution.

You must be a master of boolean !!

Good to know there are people kind enough to help others in forums. Thanks, Jscher!