Thanks for not only posting your issue, but also updating with newly discovered information. I wasn't aware that uBlock can prevent caching.
Firefox
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I have now tried this behavior on more profiles and devices (originally I tested on 2 profiles), and it it seems this behavior isn't universal.
On the same device (gentoo linux) and a laptop (windows 10) I do not see this behavior. One profile (my main profile) has many historic changes, the other though is quite new and not very unconventional settings wise. Both see this issue. The third profile on the same device is basically empty, only used for testing a few times.
However the win10 profile is a fork of my main profile from 1.5y ago, and should thus be similar in usage and configuration weirdness, yet it works just fine. The only Idea I have right now is it being related to the number of tabs open, since the two profiles seeing the issue have probably over 100 over multiple windows, while the other two only have a single window with maybe 10-20 tabs. Through opening a number of heavy tabs I could not recreate this yet. A restart doesn't fix it either, meaning the tabs in their unloaded state would still contribute if that were the cause.
I have Pinned it on uBlock Origin.
ubo seems to prevent creation of cache for pages while it is active.
Testing around with enabling and disabling addons, I saw the issue disappear. I then reenabled all addons and the issue was still gone. So I restarted and it came back. Then I disabled one after another, and got to it being ubo. I then tried disabling it, loading a "gallery page" and an entry on it, then reenabling ubo. The pages were still cached, but visiting a different entry would reload that one every time. Same for any other newly opened "gallery" and "entry" pages.
This explains why this didn't happen in the test profile, which has no addons installed, but the mirrored profile on the laptop does have ubo and ff on the same version, however I do not observe the behavior there. I will investigate further.
Actually it happens on the forked profile too now, restarting ff did not clear the cache there and it only occurs on newly opened pages now, so what I assume happened was I initially tested while ubo was not yet updated, since the laptop has been sleeping for a while. The browser generated a bunch of cache entries for my test pages and those continued working when ff updated ubo. Clearing the cache now has it break on all pages and fix itself when ubo is disabled like on the other device.
Therefore this seems like a clear ubo issue, independent of platform and other factors. All 3 affected profiles have similar ubo config, so I'll investigate that now. However it is not a firefox issue, I will move the issue elsewhere.
Hrm, I have not noticed any such change, at least not actively.
I have now found instances it doesn't happen for. It might be related to the number of open tabs, or some other indicator of recent heavy usage.
Do you have ubo installed? If not, could you try installing it and see if you can recreate it then? Specifically version 1.54.0. You'll have to find a new test page or clear cache after ubo has been enabled.
Did you find the cause?
Yes, I just updated the post, you can see the process in my various comments.
It seems to be caused by uBlock origin, though I haven't yet investigated deeper. It might be the combination of the firefox version and the ubo version, and it may be dependent on a specific setting within ubo, I don't know yet.