Strange behavior in firefox with <tab> throgh fields

Question

I have a strange behaviour in firefox (in other browsers it works fine):

I have a bunch of fields I want to tab through with the key. But unfortunately this doesn't work for all the fields. In some you can tab into but once you are there everything is stuck and you can't either tab forth or go back.

The funny thing is if I inspect the field with firebug and edit the specific input field by adding a whitespace char (or whatever else) and go back, the tabbing through the fields works fine. I really don't understand why this happens. As we are using backbone.js maybe it's even somehow related to backbone rather than firefox...

The below code shows the generated HTML output. This tabbing issue only occurs when it comes to input fields. The select section works fine.

<table width="100%" class="display bill">
    <thead>
        <tr>
            <th class="field-quantity">Quantity</th>
            <th class="field-price">Price</th>
            <th class="field-total">Total</th>
        </tr>
    </thead>
    <tbody class="list">
        <tr>
            <td class="bbf-field field-quantity numeric">
                <span class="editor">
                    <input type="number" value="1" class="" step="any" name="quantity" id="c378_quantity">
                </span>
            </td>
            <td class="bbf-field field-price numeric">
                <span class="editor">
                    <select id="c378_price" name="price">
                        <option value="4">64.00</option>
                    </select>
                </span>
            </td>
            <td class="bbf-field field-total numeric">
                <span class="editor">
                    <input type="text" readonly="readonly" name="total" id="c378_total">
                </span>
            </td>
        </tr>
    </tbody>
</table>