App specific firefox shortcut not working

Describe the bug
Remapping the "switch tabs" shortcut as shown in the screenshot below does not work in Firefox.

Affected input device (e.g. MacBook Trackpad, Magic Mouse/Trackpad, Touch Bar, etc.):
Macbook keyboard

see above

Device information:

  • Type of Mac: macOS Mojave 10.14.4
  • macOS version: 10.14.4
  • BetterTouchTool version: 2.750