There is no magic. Either the browser stores the source somewhere or the browser refuses to show source or the browser refetches the source from the server when asked to show it.
Given that in the setting of that bug report the user has explicitly instructed the browser to not store anything anywhere, and that you don't like the third option (the current behavior), I assume your proposed solution is to put up some sort of error message instead of showing the source in this situation?
I suggest to do it like Chrome does, show me the source. Maybe it caches the original source, I don't know, but I can't imagine that's expensive (given all that Firefox caches on each tabs ...).
Caching the original source can in fact be pretty expensive in a lot of cases. Which is why Firefox tries to put it on disk, not in memory. But, again, if the user then explicitly turns that off, then Firefox doesn't do it.
The key part here is that this bug report is about a situation in which the user explicitly changes the browser's default settings to not store stuff. And then the browser ... doesn't store stuff. Shocking, I know.
Given that in the setting of that bug report the user has explicitly instructed the browser to not store anything anywhere, and that you don't like the third option (the current behavior), I assume your proposed solution is to put up some sort of error message instead of showing the source in this situation?