My attempt to demonstrate the issue described in Bugzilla bug 1890697.
The Firefox debugger often attempts to fetch fresh copies of sources, especially if the debugger was not open when the parent page was initially loaded.
It also appears to perform GET requests for sources which were originally acquired via POST requests.
This little site attempts to make it easy to see this behaviour in the real world by generating identical responses for both GET and POST requests.
When trying to reliably replicate this I've found that - with an empty cache - if the debugger is opened within about 5 seconds of the initial page load a new request is not made…
Debugger should show sources as they were when the page was loaded, or not at all. Opening the debugger should not result in any new network requests (except perhaps to retrieve source maps).
The swapping of POST for GET is a definite no-no.
I work on an ancient web application which doesn't use reusable URLs or the Post/Redirect/Get cycle. Making a second GET request to the same URL, or swapping HTTP verbs for a given URL can result in wildly different results. I therefore cannot use the Firefox debugger when working on this application and that makes me sad 🙁
If the server times are different this indicates the debugger made a fresh request for the page source.