You only need the cache when it is needed. That means it is no part of your profile files - it gets created and removed anyway. Moreover, if you would copy the cache (i.e. the Local path’s files) to another system, it may be useless, not work or not even get removed. There is no need to worry about it - that’s why you probably won’t find any instructions to back up the cache, but rather to remove it instead, e.g. when suffering install issues, at most. You can consider this to apply both to Firefox and Thunderbird.

The menu button ≡ should show a question mark at the bottom right in Firefox next to the Exit/Quit icon. It has no capitalized text-based Help entry (but like all items there is a tooltip), though that will return in newer Firefox versions. In Thunderbird, there is a menu item called Help at the bottom of the menu that gets opened when clicking ≡. Of course, you can always use the Help item in the top menu bar that may be hidden - press Alt or F10 to display it or, when at it, enable View > Toolbars > Menu Bar in the same menu bar to keep that displayed.

Does this answer your questions?

Well, this does reinforce the statement in the article "Which backup do I use Local or Roaming", but I still would like to know what AppData\Local\Mozilla\Firefox\Profiles is for. Its "Properties" box indicates that it consumes 359 MB and has 26,058 files and 26 folders; that seems a lot for a folder that's useless and might not run if restored to another system.

I'm even more concerned about Thunderbird, which has messages going back to 2005; here the corresponding Profiles folder in Local takes up 260 MB and contains 658 folders and 5,885 files. Do I really want to risk skipping it in my backups and so losing all that data if my hard disk crashes irretrievably?

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

https://addons.mozilla.org/en-US/firefox/addon/febe/ FEBE (Firefox Environment Backup Extension)

FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backups of files that you choose.

https://addons.mozilla.org/en-US/firefox/addon/opie/ OPIE

Import/Export extension preferences

The cache is a tempory file where data from websites is stored. If removed or bypassed, the data is downloaded from the website.

It does reinforce the statement in "which backup do I use Local or Roaming" question you referred to allright, but I suggest not to doubt what is said there. In general, it’s best not to argue with instructions or advice with regard to Firefox/Thunderbird profiles. If you like running into issues, back up your cache folder, restore it on another machine and play with it in a test profile. You won’t win anything, if it works in the first place.

As said, there are better ways of getting cache removed if you like to gain disk space. There is no single use in backing up cache files - OSes don’t to it either, in most cases, or recreate them too. And even if it did work, it would be better to let them recreate than using old and polluted ones.

For Thunderbird, you don’t need the cache files either. The messages are what counts - some of the cache is web cache, and message cache is recreated anyway. I really see no point in your doubts other than that you like to be involved with non-existing issues "on file level" like in the old days. I do agree modern OSes and applications cause a lot of temporary files and tend to ignore disk space, but would expect concerns about "how to remove obsolete files in backups" instead for that reason. ;)

The location in "AppData\Local" is only used for storing temporary data. This location also stores data for the thumbnails for the about:newtab page and data for the safe browsing feature (malware and phishing protection) as you can see if you inspect this via the about:profiles page and go up one lvevel.

Thanks, cor-el! You're the first respondent who actually addressed the question!

If the Local profiles are needed for malware and phishing protection, though, shouldn't they be backed up? I certainly want such protection.

I don't see what you mean about going "up one level" from about:profiles. That screen allows me to open either the Local or the Roaming profile, and I certainly can go up one level from either, but I don't find that doing so tells me anything about thumbnails or safe browsing.

Thanks for your input re FEBE, FredMcD, but I prefer to use File Explorer for backups, a program I'm familiar with that has Microsoft behind it.

I do not have a disk space problem. All the same, in backing up, I would prefer to omit steps that are pointless, and the consensus seems to be that backing up the Local profiles falls into that category.

The question remains: why is my computer saving 31,943 files (Local Firefox and Thunderbird profiles together) if they're not of use for future processing? And if they are of use and future processing will take place on a new computer (mine is 12 years old), don't I need to back them up so that they can be restored on a new machine?

I don’t get why this is so hard to comprehend. Apart from a detailed explanation of what AppData\Local is for (something you can find outside of this forum), you were told instantly and more than once that they are volatile files, and there would be no need to back them up, or they could cause fails when doing so when transferred to another machine.

Do you know how a turbo charger works in an engine? Simply put, it reuses gasses from the exhaust to drive a turbine to spin an air compressor that pushes extra air into the cylinders in order to improve performance.

Do you think such engine keeps that pressure built up last when the engine is turned off, or would you think it will build up new pressure at the next start, causing the turbo to work a number of seconds later?

That is similar to how local disk cache works. You could keep them, but basically, it would be a waste of resources - if it is about backups. Otherwise they will be used (and refreshed) by Firefox at every use, but your question was about backups.

cor-el says that the location in AppData\Local is used not only for storing temporary data but also data for the safe browsing feature. I certainly want safe browsing, so why should I abandon the data it needs?

Tonnes presents an analogy to an engine with a turbo charger, viewing engine pressure as analogous to disk space in Local. "Do you think", he asks, "such engine keeps that pressure built up last when the engine is turned off, or would you think it will build up new pressure at the next start"? The latter, of course; when the engine is turned off, it releases the pressure. In a similar way, if I turn off my computer or just exit the application, I would expect that the disk space would be released. That's not what happens, however. I just turned my computer off and on again; without my opening anything but File Explorer, I find that the Firefox Profiles in Local occupy 403 megabytes on disk, composed of 27,205 files and 26 folders. Is Firefox just too lame to release that space so that other applications can use it?

Amongst other purposes, the cache files are also used for the Safe Browsing feature allright, but that does not mean things break if you delete or not include them in a backup. If not cached, they would need be retrieved for every website the browser checks its entry in the Safe Browsing list. Hence the original list and files that are needed will be retrieved for any new Firefox install (or at least after using a new profile for the first time), that’s why you will see more disk activity at that point. Better yet: such files for that feature are refreshed numerous times, which can even happen within one Firefox session. Perhaps you should launch the Windows Resource Monitor and have a look at the disk activity section to see what happens with cache files related to the Safe Browing feature and other tasks.

You do understand the concept of cache, don’t you? It is used to retrieve data from a local disk (or memory) rather than retrieving it from the network in order to a) save data use / bandwidth and b) possibly speed things up, period.

A difference between a turbocharger and a computer program is that the first would need a bunch of technology for keeping the pressure previously built up in order to save 5 seconds of useless performance loss right after starting a car, while a computer program can just store cache files (which happens anyway) and reuse them after a relaunch of the program or even a restart of the OS, but I assumed you would understand that. Every application and even OS uses it, that’s why you will never find empty cache folders - no wonder some applications or OSes sometimes forget to remove cache here and there. Backups are intended for primary files that contain actual and important data while maintaining full functionality - cache is not considered to be part of it, and therefor hardly ever documented. Frankly or for what I can remember, you are the first person in about 30 years that I came across and ever begged for an answer on why (not) to preserve cache content in a backup. ;)

You may do good by not asking new questions and accepting the fact that people here are trying to explain how things work and not trying to talk rubbish. I would consider the question to be solved, so please mark a solution when you agree and are ready for it.

Torres, I probably should have phrased my original question in terms of profiles rather than caches. What I have been backing up are the Firefox and Thunderbird profiles in Roaming, and my concern really was whether the profiles in Local should be backed up as well. I note that, except for "thumbnails" and "safebrowsing", the files in the Local profiles are all caches, as implied by their names, while the Roaming profiles contain substantially more folders, none of which call themselves caches. Since you've addressed safe browsing, I'd at most lose the thumbnails by failing to back up the Local profiles, and perhaps not even these.

My thanks for all your time.

All data stored in AppData\Local is temporary data and Firefox will automatically redownload safe browsing data and create new thumbnails for the about:newtab page and repopulate the other folders if this folder would been removed, so there really is no need to backup this folder. If you would backup and restore safe browsing data then you would restore outdated data that would no longer apply.