Starting in Firefox 14, Firefox no longer creates registry settings for launching links using "DDE". This has been reported to create two different problems: * "Firefox is already running" or "Firefox is not responding" error when following links in email messages in Thunderbird ([https://support.mozilla.org/en-US/questions/932461 Can't open links in Firefox by clicking URL in Thunderbird]) * New instance of Firefox launches if the existing instance is running in Sandboxie ([https://support.mozilla.org/en-US/questions/933798 Firefox is running. I click on a link, say in an email, it tries to start another instance of the browswer instead of just opening a new tab as it did before.]) Not sure whether your "silent fail" scenario is related. It is possible to configure Firefox 14 to use DDE by inserting some settings into the registry. I haven't researched this carefully myself, but there was some information about it on and external site linked in the Sandboxie thread.