Neither VisualEditor nor the 2017 wikitext editor honors Firefox’s ui.key.contentAccess preference. For example, I set it so that Control-F focuses the search box instead of Control-Option-F. This preference works with any accesskey in MediaWiki, including the Save button in the old wikitext editor, but not the Save button in VisualEditor. Instead, VisualEditor’s Save button responds only to Control-Option-S, which would be the standard key binding on macOS and the default in Firefox. This is admittedly an edge case, but I customize the accelerator because I find any Control-Option combination to be awkward to type.
Yeah, I think the only way to respect the setting would be to use the accesskey HTML attribute or DOM property, as most of MediaWiki does. (Manually binding to hardcoded key combinations using event listeners would lead to this issue, for example.)