User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:55.0) Gecko/20100101 Firefox/55.0 Build ID: 20170814072924 Steps to reproduce: 1. Menu > Options. 2. Press the accelerator key associated with any option. Example #1: on the General page, press Alt-K to toggle 'Show tab previews in the Windows taskbar'. Example #2: on the General page, press Alt-H to toggle 'When you open a link in a new tab, switch to it immediately'. Actual results: In most cases, nothing happens (e.g. example #1). In other cases, pressing the accelerator triggers an item on the main menu (e.g. example #2). Expected results: The appropriate action should be triggered. Example #1: on the General page, pressing Alt-K should toggle 'Show tab previews in the Windows taskbar'. Example #2: on the General page, pressing Alt-H should toggle 'When you open a link in a new tab, switch to it immediately'.

Now that the page is in-content, you need to add Shift to the accelerator key combination. So it would be Shift+Alt+K or Shift+Alt+H in your examples, respectively.