In case an extension or cookie setting is causing this issue, could you try this test: use the site in Firefox's Safe Mode in a Private Browsing window. This will help bypass restrictive cookie settings, as well as other filtering behaviors of extensions, to see whether those are a factor.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the small dialog that should appear, click "Start in Safe Mode" (not Reset)

Then open a private browsing window using either:

• Ctrl+Shift+p
• File menu > New Private Window
• orange Firefox button > New Private Window

Then log in to the Add-ons site and try the edit again. Any difference?

If it still doesn't work, I suggest trying their forums over there since it could be a site bug: http://forums.addons.mozilla.org/

Thank you for answering so fast, but the Safe Mode couldn't fix the problem. I'll try to get more information at the forums, but thanks at all.

You should be able to upload a new version of a theme. Have you tried that?