Hello Blea, I looked hard into implementing a settings page that would allow you to customize the shortcuts to whatever you like however as of today (57.0.1) this NOT SUPPORTED by Firefox, as confirmed here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1303384

for the moment I think what you could do is unpack the extension and edit the manifest.json to specify your own preferred shortcut and then "sideload" it from about:debugging.