If the previews are part of a website, could you provide a link to the page that isn't working correctly? Also, I will paste some general suggestions below. If the previews were part of a video download list, do you know which add-on it is? Some of them may need to be updated for Firefox 26 and late due to the changes in the way downloads are listed. ---- 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) Bypass Firefox's Cache Use Ctrl+Shift+r to reload the page fresh from the server. Alternately, you also can clear Firefox's cache completely using: "3-bar" menu button (or Tools menu) > Options > Advanced On the Network mini-tab > Cached Web Content : "Clear Now" If you have a large hard drive, this might take a few minutes. (2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either: * right-click and choose View Page Info > Security > "View Cookies" * Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies" In the dialog that opens, you can remove the site's cookies individually. Then try reloading the page. Does that help?