iron-flex-layout demo freezing in firefox

[lingtalfi]

Description

The demo for iron-flex-layout makes firefox crash (unusable to be more precise).

https://elements.polymer-project.org/elements/iron-flex-layout?view=demo:demo/index.html&active=iron-flex-layout

Expected outcome

go to the page: https://elements.polymer-project.org/elements/iron-flex-layout?view=demo:demo/index.html&active=iron-flex-layout
with firefox and see a demo.

Actual outcome

The page seems to perform a very heavy computation that never ends.
The only way out is to force quit Firefox, even then, after restarting, Firefox is still slow every time when a iron-flex-layout page is approached in the polymer elements section (https://elements.polymer-project.org/elements)

Steps to reproduce

1. Open the above url in Firefox

Browsers Affected

• Chrome
• Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10

[notwaldorf]

I can't reproduce this in the latest Firefox :(