On Thu, 19 Dec 2019 at 11:24, Peter Wolf via luv-beginners <luv-beginners@luv.asn.au> wrote:

My computers are 10-20 years old and not overheating except for maybe 1 or 2 out of 20-30 computers.

- After visiting dodgy websites that create dozens of connections to residential computers with Firefox,

I'm getting mostly the same thing on multiple computers (more than 10),they do have old hard (disk) drives.(5-15 years old) The above trouble has 90% gone after using my custom software to "stop the hacking".It's an interesting suggestion and although I very much doubt it is the cause, I can't definitively say that it's not.So I'll make a point of checking my hard drives.

These are the most telling statements.

10 to 20 year old computers are likely to struggle to efficiently run the latest version of Mint with its v4+ (?) kernel.

5 - 15 year old disks will be well used by now and very likely to be carrying bad sectors.

It is likely that the OS has detected this and is starting in a read-only mode explaining things like the permission issues you are seeing and the apparent data loss i.e. it's not getting stored in the first place. This would also contribute to performance issues.

Now to ease your concern about Mint's default security, I installed a fresh version into a virtual machine, did the base level updates, then installed nmap (a network port scanning tool) and scanned the local host thus...

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tfeccles@mint-mate-vm:~$ sudo nmap -sS localhost

Starting Nmap 7.60 ( https://nmap.org ) at 2019-12-19 12:16 AEDT
Nmap scan report for localhost (127.0.0.1)
Host is up (0.0000060s latency).
Not shown: 999 closed ports
PORT STATE SERVICE
631/tcp open ipp

Nmap done: 1 IP address (1 host up) scanned in 1.62 seconds

--------------------------------------------------

As you can see, the only port open is the CUPS printer port.

I enabled the firewall via the control centre, activating the Home profile, re-scanned and got the same results.

Unless you've tinkered in some way to weaken the OS and browsers, I'd say, with a close up hands-on look, most of your issues are hardware related.