Jamncream on "[Plugin: WooCommerce Collapsing Categories] Increasing indents with Firefox"

Hi, I am currently testing this plugin, which works well in Chrome, but has increasing indents for categories as the list goes down.
I would really like to use the plugin and will happily donate if you can please assure me this can be rectified?
My test site is at:
http://www.jamncream.com/themes/1/about

It displays as expected in Chrome and IE, but not in Firefox (45.0.2)

Could you please advise how this additional indentation can be eliminated?
I am using a custom theme, but I tested with "Twenty Fourteen" theme and it behaves the same way (extra indentation as the list goes down).

Regards,
Jam

https://wordpress.org/plugins/woocommerce-collapsing-categories/

If we can sort that issue, I'd REALLY like to replace the "-" and "+" with an image (I can produce the image). Would that be possible?

Following up, if I disable "Load mTree" in settings, then the extra indentation is removed, but the symbols - and + are also removed.