I think the ~/.mozilla/firefox/XXX.default-YYY/storage/default/https+++ZZZ.com/cache/https+++domain.com/ style dirs are the storage for what's called "service workers" which is persistent code related to each website that sends notifiications even if no related tab is open.
Suppose you have a favorite website that sells something, you might register with them that you're interested in a particular kind of product. A serviceworker for that site would be in the "ZZZ" folder named after that site, the code in there would run even if you don't have a tab open for that site. It's done so you can get a notification. In other cases it's some other code that the web designers don't want to have to reload each time you visit, caching it in your storage folder saves time and network.
You can see all your service workers in the Firefox menu: Help -> More troubleshooting information -> about:serviceworkers ( or load about:serviceworkers )
Start Time is the time from start of 1st request until the start of this request.
End Time is the time from start of 1st request until the end of this response.
Latency is the time from end of this request until the beginning of download of this response.
Response Time is the time from start of 1st request until the beginning of download of this response.
Duration is the time from start of this request until the end of this response.
Every modern web browser includes a powerful suite of developer tools. These tools do a range of things, from inspecting currently-loaded HTML, CSS and JavaScript to showing which assets the page has requested and how long they took to load. This article explains how to use the basic functions of your browser's devtools.