Starting from Windows Fall Creators Update 1709, cookies and history are no more stored in files, but in a database named WebCacheV01.dat under AppData\Local\Microsoft\Windows\WebCache
This is probably a jetblue/ese database, but it is apparently constantly locked by the OS and can't be accessed normally.
Single cookies can be accessed through the Internet(Get|Set)Cookie(A|W) APIs, but there isn't an API to get a list of cookies.

This means our cookies migrators for Edge/IE are not working, or, in the worst case, they are migrating really old leftover cookies created before this version.

A possible alternative may be to "try" reading cookies for a specific list of urls (the ones imported with history/bookmarks) using the APIs and ctypes, though that's not a trivial option.

Considered the IE/Edge market share, the fact Edge is moving to Chromium (so it's unclear what's the future of cookies storage) and the relatively small benefit migrating cookies gives us (a little bit more streamlined migration on certain websites), I am wondering if it's still worth trying to keep up with this cookies migration code.

Matt, any idea who may be a product person to evaluate this decision?

Maybe Asa or pdol?

Note that cookies import from Chromium is also kinda broken. See the bug links in the cookies row of https://wiki.mozilla.org/QA/Firefox_migrators#Supported_data_types

We have also had issues in the past caused by importing cookies because the cookie could be tied to info that is browser-specific e.g. related to feature detection / UA sniffing. While it can give a really smooth onboarding experience, when it works, I think importing passwords would be safer but still help users log in to sites. I'm not sure that cookies used for preferences, not logins, are as essential to users.

I've heard via email from Asa, saying "I'm OK with removal. I don't think cookies are as key a data source as some of the others"

Done in bug 1801051.