Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0
Build ID: 20190225143501

Steps to reproduce:

• open the preferences/options panel
• go to the "Cookies and Site Data" tab
• ensure that "Delete cookies and site data when Firefox is closed" is checked
• go to https://send.firefox.com/

Actual results:
the site is displayed but the main area is blank: main site feature is broken

Expected results:
the site should work as expected

There is the following error in the console:
"Failed to register/update a ServiceWorker for scope ‘https://send.firefox.com/’: Storage access is restricted in this context due to user settings or private browsing mode."

Please report this to https://github.com/mozilla/send/issues

Somebody else already reported it: https://github.com/mozilla/send/issues/1226

However, setting the option to "Delete cookies and site data when Firefox is closed" should not change the browser behavior during my session but only delete cookies and site data on exit.

Can you explain to me why enabling some cleanup when closing the browser suddenly prevents a site to register a service worker ?

There is clearly something fishy on the browser side.

May I be so bold to ask you to either give me some explanations why this bug is invalid or reopen it, please.

It's not fishy but currently on purpose that service workers are disabled based on the cookie settings.

So it is done on purpose, that's good to know. Thanks.

And the Send team is implementing a fallback when ServiceWorker are not available.

I do want to point out that nothing on the setting's label convey that change of behavior when checked.
Neither is there a SUMO link to explain it.

It is a setting surfaced on the preference page, easily changeable by any non tech-savvy users, that can break web sites without said users understanding the relation. My mother knows what a cookie is but has no clue to what a ServiceWorker is or does. For these users, this 'hidden' behavior associated to this setting is kind of a trap, that it is done on purpose does not change that fact.

I invite you to have a look at the few comments on the Send issue (https://github.com/mozilla/send/issues/1226):
"[...] the problem seems to be in what that option does. It no longer does what it says, just "Delete" cookies without actually changing them."

I'm just a bug report triage guy and only explained the current situation.
Please redirect your comment into bug 1413615 where the developers will see your comment.