When you adjust the zoom level using the buttons and dropdown in the toolbar button popup, the user-defined zoom levels will always be applied.

When you adjust the zoom level using the Ctrl+- and Ctrl++ keyboard shortcuts, the user-defined zoom levels will be applied if the page contents are focused, but the Firefox default zoom levels will be applied if the page contents are not focused. It is not possible to change this behaviour.

It would be possible to have separate keyboard shortcuts (e.g. Ctrl+Shift+< and Ctrl+Shift+>) that would always apply the user-defined zoom levels, but this might be confusing for users. Any thoughts?