Offset is incorrect with table in firefox

If Ember sortable is used within a table in firefox the item being dragged jumps around 200px.

## Behaviour on Firefox

## Behaviour on Chrome

## Code to reproduce
```

{{#sortable-group tagName="tbody" onChange="reorderItems" as |group|}}
{{#each items as |item|}}
{{#sortable-item tagName="tr" model=item group=group handle=".handle"}}

{{/sortable-item}}
{{/each}}
{{/sortable-group}}

{{item.name}}


↕

```
```
import Component from '@ember/component';

export default Component.extend({
items: [
{ name: 'Test 1' },
{ name: 'Test 2' },
{ name: 'Test 3' },
{ name: 'Test 4' },
{ name: 'Test 5' },
{ name: 'Test 6' },
{ name: 'Test 7' },
{ name: 'Test 8' },
{ name: 'Test 9' },
{ name: 'Test 10' },
]
});

```