In https://bugzilla.mozilla.org/show_bug.cgi?id=1524200#c41 :baku proposed:

Probably, it's time to have a different approach. I suggest to introduce a cleanup dialog, to be shown after the closing of firefox. In this dialog we can have a progress bar, and maybe a list of operations that the browser is performing (cleaning X, cleaning Y,...)

This dialog would be shown only when needed: to the users who want to delete data on shutdown, or/and to the users who have set a custom session-only cookie permission for some origins. This dialog could be also used by other features such as 'forget about site' and 'clear data history'.

I have already discussed this proposal with johannh. I will organize a meeting to discuss it with more people.

I don't think a spin-off bug was ever filed for this and I haven't found evidence that we have such a dialog (but I would be very happy to find out I am wrong about this! :), so I'm filing this bug.

In particular, in bug 1413615 we could probably enable Service Workers when "delete cookies and site data when Firefox is closed" is enabled as already implemented, but enabling Service Workers could potentially cause hundreds of additional megabytes of and thousands of additional files to be synchronized (ex: slack.com) which would have a very noticeable impact on shutdown as data clearing is currently implemented. (Note that there are very non-trivial enhancements to Cache API storage that could be made to accelerate shutdown, but I'll discuss those there.)

Note that the dialog should also do something to make sure that the shutdown watchdog timer doesn't fire and trigger a crash while forward progress is being made on data clearing.