(using Firefox Quantum on Windows 7) As the title says. I have google sheets and I have allowed "Override browser shortcuts" on the settings, so when I press ctrl+shift+pgdown (and ctrl+shift+pgup) it should change sheet, what it does instead is to move the firefox tab. It looks like this specific command is not overridden (it hasn't happened with the others that I'm aware of). On Google Chrome this behavior works as expected. This is problematic since I use multi-sheets documents on a daily basis, and as a mainly keyboard user this is strongly limiting my capabilities with the software.

Thank you for your patience!