Created attachment 353900 [details] screenshot: install dialog Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1b3pre) Gecko/20081219 Calendar/1.0pre Third button without label in add-on install dialog of add-on manager (see screenshot). Steps to reproduce: 1. Open Add-ons Manager. 2. Open http://ftp.mozilla.org/pub/mozilla.org/calendar/sunbird/nightly/latest-comm-central-sunbird-l10n/ and drag and drop a language pack into the add-ons manager. Actual result: There is a third labelless button between "Install" and "Cancel".
Works for me using Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1b3pre) Gecko/20081219 Calendar/1.0pre (BuildID: 20081219033507). The dialog contains only the "Install" and "Cancel" buttons. The "Override 'maxVersion' compatibility checking" checkbox is not part of the default dialog. I assume you are using some extension that changes the dialog and causes the issue. Please remove the extension and test again.
Sorry, my fault. Caused by Mr Tech Toolkit (was so used that all extensions are disabled on trunk). Mailed the developer.