When the bookmark item is removed the id
will no longer be useful. The onItemRemoved
method takes more arguments than the three that you mentioned though, it takes aId, aParentId, aIndex, aItemType, aURI, ...
so you can use the aURI argument to get the url that you are interested in.
The better option is to use the sdk/places/events
module though, like so:
const { events } = require('sdk/places/events');
events.on('bookmark-item-removed', ({ data }) => {
let url = data.url;
// ...
})