Today I noticed that in the Page Info --> Permissions tab (which you access through padlock --> Connection secure --> More information beside a page's URL) there is an Override keyboard shortcuts permission setting, with default set to "Allow". Interesting, because some sites use shortcuts that I'd rather avoid.
I checked the Settings, but I see no entry to set the default to "Block". Is there some entry in about:config for this?
Cheers!
Thank you so much, that did it!! I hope they'll put this option explicitly in the settings, in the future.