You probably noticed a change in Firefox 38, switching from the pop-up Preferences dialog to a page-based system. Could you test Preferences in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: [[Troubleshoot Firefox issues using Safe Mode]]. ''If Firefox is not running:'' Hold down the option key when starting Firefox. ''If Firefox is running:'' You can restart Firefox in Safe Mode using either: * "3-bar" menu button > "?" button > Restart with Add-ons Disabled * Help menu > Restart with Add-ons Disabled and OK the restart. ''Both scenarios:'' A small dialog should appear. Click "Start in Safe Mode" (''not'' Refresh). Does the Preferences page work? ---- If we aren't able to quickly spot the issue, you could try switching back to the old dialog for now. This is how: (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 '''pref''' and pause while the list is filtered (3) Double-click the '''browser.preferences.inContent''' preference to switch it from true to false Does that make any difference to whether the checkbox is enabled? (Note: I don't know whether the old dialog will be in Firefox forever or is a transitional feature. As you've probably noticed, the long-term trend is to remove things that pop up.)