Firefox Error while fetching Database list

After upgrading from version 0.29.0rc1 to 0.999.0dev. Firefox doesn’t load the welcome screen and gives errors when opening SQL Lab editor or any dashboard "Error while fetching database list " "Error while fetching schema list "

SQL lab queries are executed as expected Single charts do render. but not when included in a dashboard.

Chrome and Safari work correctly

Expected results

After logging in my dashboard renders, and sql lab editor has a database and shema dropdown lists filled. No errors are displayed.

Actual results

Welcome and Dashboard screens keeps loading.

SQL Lab editor dorpdown list aren’t filled. auto completion doesn’t work

No errors are written to the logs

Screenshots

The welcome screen that keeps loading

Initial load of the SQL Lab editor

How to reproduce the bug

1. Log in

2. Wait until you give up

3. go to SQL Lab Editor

4. try to select a database in the dropdown

Environment

(please complete the following information):

• superset version: master/ 0.999.0dev.
• python version: Python 3.6.8
• node.js version: v10.15.3
• npm version: 6.4.1

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• [x ] I have checked the superset logs for python stacktraces and included it here as text if there are any.
• [x ] I have reproduced the issue with at least the latest released version of superset.
• [x ] I have checked the issue tracker for the same issue and I haven’t found one similar.

Additional context

Watching network trafic in the browser, it doesn’t seem like firefox does any of the data requests that I am getting in Chrome.

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[amravazzi]

+1.

I got multiple errors while loading the Superset through an iFrame in my application: