MagicIsInTheAir said

Firefox proceeds to clean/delete the cache files at certain time intervals, the assigned size limit in the setting outlined below do not seem to affect this behavior. This forces me to reload some websites with allot of content to go through because the websites fails to read the cached content as I progress, as those cached files have been deleted by Firefox.

Hi MagicIsInTheAir, if a file is completely missing from the cache, Firefox should instead retrieve it from the server. If something is not loading correctly, perhaps a cached file is obsolete or corrupted in some manner?

More generally:

I thought files would be cleared on a Last-in-First-out basis when needed to make more room, so actively used sites should be the least affected. However, I haven't researched it in detail in recent years.

Files also will be ignored or updated if they have passed their expiration dates, which are based on the headers sent by the server. You can view expiration dates in about:cache along with the URLs. This is not the most organized page. For testing, you might want to create a separate profile (each profile has its own cache folders). I'll describe the steps or that at the end.

Also, does your Firefox ever have an abnormal shutdown? In the old days, the entire cache was discarded after a crash. However, I don't know whether that is still the case.

Test Profile

This takes about 3 minutes, plus the time to investigate problem site(s).

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the "Create a New Profile" button, then click Next. Assign a name like Sept2019, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

Does caching work any better in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Sept2019 will remain available for future testing.)

jscher2000 said

I thought files would be cleared on a Last-in-First-out basis when needed to make more room, so actively used sites should be the least affected. However, I haven't researched it in detail in recent years.

Hi jscher2000, and thank you for your quick and thorough reply!

I went ahead and created a new profile and tried to use it for a shorter amount of time and it seems to work exactly as you described it above, Last-in-First-out. I suspect an addon might have been the problem. I'll disable the addon in my old profile. I'll test it for a couple of days and report my findings.

Cheers!

Are you possibly using third-party cleanup software that could be responsible ?

cor-el said

Are you possibly using third-party cleanup software that could be responsible ?

Hi cor-el, thank you for your response. I'm not using a third-party cleanup software.

uMatrix, a privacy addon was periodically cleaning my cache, it has a setting under the Privacy header called:

Clear browser cache every N minutes

That setting seems to be enabled by default.

jscher2000 suggestion led me to the solution, thank you jscher2000. Since I'm posting the solution to this particular problem, I'll mark my post as the answer.