Tabbing between paper inputs broken on latest Firefox

[umbertoand]

Description

Tabbing between paper-inputs seems to be broken on recent firefox versions:

• On firefox production, tabbing and shift-tabbing seem to be broken.
• On firefox nightly, only shift-tabbing is broken.

Expected outcome

Shift-tabbing should return focus to previous element

Actual outcome

Either returns focus to window or previous block of content

Live Demo

This is actually visible on the paper-input demo on webcomponents.org

Steps to reproduce

1. Go to https://elements.polymer-project.org/elements/paper-input?view=demo:demo/index.html&active=paper-input
2. Give the second paper-input (the password one) focus
3. Hit shift+tab

Browsers Affected

• [ ] Chrome
• [x] Firefox
• [ ] Safari 9
• [ ] Safari 8
• [ ] Safari 7
• [ ] Edge
• [ ] IE 11
• [ ] IE 10