Is this a plugin or an extension? The dialog you're describing sounds like an extension. Firefox 43 disables extensions that have not been digitally signed by the Mozilla Add-ons team. To check whether this is the problem, please go into the Add-ons page. Either: * Ctrl+Shift+a * "3-bar" menu button (or Tools menu) > Add-ons In the left column, click Extensions. Then scroll the list on the right side until you can see the bottom, where disabled extensions are listed. If Slingbox is there, does it mention not being verified? If so, that probably is the issue. This article has more information on extension signing: [[Add-on signing in Firefox]]. I'm not sure whether there is an update, but if there isn't or if it isn't possible to install it, you can make a temporary exception to the signing requirement until that update is available/installed. This is global for all extensions, there's currently no way to make an exception only for this extension. (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''xpin''' and pause while the list is filtered (3) Double-click the '''xpinstall.signatures.required''' preference to switch it from true to false -- note that this is for '''Firefox 43 only'''

Thank you so much for your help. Much appreciated.