Make sure that you do not run Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

Previewing a lightweight theme (Persona) only works on sites that are white-listed via an allow exception, so make sure to have an allow exception for addons.mozilla.org domain and possibly the the getpersonas.com domain.

You can see the permissions for the domain in the current tab in:

• Tools > Page Info > Permissions > [X] Install Extensions or Themes
• Firefox > Preferences > Security : "Warn me when sites try to install add-ons": Exceptions

Make sure that you do not use Clear (Recent) History to clear the "Site Preferences".

• https://support.mozilla.org/kb/Clear+Recent+History

I am not using private browsing mode. What would that have to do with the subject anyway? I also never clear the site preferences. I am not using personas either, but full-fledged themes. I am not talking about previewing on websites, but from the Add-ons window with themes that I already have installed. The preview I mean is the preview.png image in the root of the .jar file for the theme. Before FF 21, I would see the screenshot for a theme in the "More" pane, along with the other information about the theme. I want to know why I do not see that any more.

Does nobody else ever look at the "More" information for a theme, or open .jar files to edit something in there?

I'm not using themes, so I don't know what you are referring to.

Can you attach a screenshot that show this more button and post a link?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.

Here are 2 screenshots. The first shows the Appearance pane where the More buttons are. The second shows the information you get if you click one of the More buttons.

Try to delete the addons.sqlite file in the Firefox profile folder.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder

It is also possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar ("Find Updates" button in older Firefox versions) to check if there is a compatibility update available.

There is no addons.sqlite file in my profile. I removed the other files. That did not help. I guess Firefox itself has been blocked from displaying the preview.png files. Thansk for trying, anyway.

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

A new profile did not make any difference. Firefox 20 had exactly the same extensions and themes as 21, and it did show preview images.

Incidentally, addons.sqlite is not used any more by Thunderbird either, and Thunderbird, being version 17, still shows theme preview images.