That is a good question.

Firefox creates per-website folders for some offline storage, but unfortunately the data is compressed, so it is difficult to read without going through Firefox's user interface. If there are sites whose storage doesn't clear, you could delete the site's folder. Here's how to find it:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows File Explorer.

Double-click into the storage folder and then into the default folder. You will find multiple folders for websites (starting with http or https) and add-ons (starting with moz-extension) and built-in pages (starting with about). If there are sites that you want to clear or just never plan to use again, I think it's safe to delete their folders, but you might want to exit out of Firefox first in case a file happens to be in use.

In the case of the NYTimes:

I notice that most of the megabytes are here:

[profile.folder]\storage\default\https+++www.nytimes.com\cache\morgue

The name implies that these contents are obsolete. Not sure why they haven't been buried yet...

When I use https://inloop.github.io/sqlite-viewer/ to look at the index of morgue contents in caches.sqlite, it looks like mostly graphic assets, from map tiles to images, and script files. You can use this query to run out a list of the source URLs --

SELECT request_url_no_query AS source_url FROM 'entries' LIMIT 100

-- but I think it's impossible to know whether any of them are actually valuable going forward.

As for why they are even there, I don't know. HTML 5 was meant to allow "offline applications" and some sites do take advantage of that feature. Maybe it's that?

Hi Jefferson, thanks a lot for this detailed answer, in particular WHERE these data are stored.

Two remarks :

1) You wrote "The name implies that these contents are obsolete". I don't understand why you say that, maybe something I do not fully uderstand in the wording (I'm french, and my english is not perfect...), but personnally I'm sure that it is NOT obsolete, since - after full cleaning of storage, any access to www.nytimes.com recreates all these data from scratch. They have not "not been buried yet", but "very recently created" - and we have the same structure storage\default\[https+++....]\cache\morgue for all websites having data in [profile.folder]\storage

2) More interesting (IMHO) : The subdir "cache", and the fact that it is mainly graphic data and scripts, may lead us to suppose they are (at least are similar to) cache data. But if it's true, why are they there and not in the "Cached Web Content" [profile.folder]\cache2 ??? And not managed the same way than other "cache content" ??? There is surely a reason (and even maybe a good reason...), but I cannot imagine which one...

Actually, maybe I can see one reason : Stored there and this way, they are fully part of the profile, and if we transfer the profile to another Pc/Laptop, they are moved with the profile, contrary to "ordinary" cache data....

Sorry, I was reacting to the word "morgue" which reminds me of a place full of dead bodies. As you point out, some of the files are quite recent, so that interpretation doesn't make sense.

I'm not sure where we can find more information about the lifecycle of those files and how to clear them on a site-by-site basis from within Firefox.

... and how to clear them on a site-by-site basis from within Firefox.

This can be done in Settings/Privacy/Cookies and site data/Manage data.... but this do NOT delete ONLY these data, but at the same time cookies, local data, indexDB, etc... relative to the website.

So the question (beyond the initial question : what are and why these data) is "how to clear them BUT ONLY THEM on a site-by-site basis from within Firefox