Downloading, screen capturing, and opening browsers all stopped working

Sorry for the length—thanks for reading, though.

Been having this issue lately: while using my Mac desktop and using my Firefox browser, I suddenly cannot download anything. It just says “Failed” next to the file in the download window. When I try to screen capture the image I get a notice saying I don’t have permission to save screenshots. When I close down Firefox and try to open it again, I get a pop up telling me a copy of Firefox is already open and I can’t open another copy of the browser. When I check the Force Quit window Firefox is not listed, and the dock shows Firefox is not open or running. When I try opening another browser I get the same message telling me that browser is already running so I can’t open another instance of it. When I tried saving a photoshop file it wouldn’t let me because of a “scratch disk error” (I think that’s what the popup said), and that my preferences won’t be saved.

My first research lead me to restarting in Safe mode. Didn’t help at all. Computer moved even slower, and browsers either didn’t display sites correctly or the browser crashed before I could use it. My next attempt was to try and rename the xulstore.json file (read that suggestion somewhere on these boards). Got a popup saying I couldn’t rename the file because there were too many letters or some special characters and to try again (neither was true). The only thing that worked was restarting the computer in normal mode, opening up Firefox and finding the Profiles folder, shutting down Firefox again, then renaming the json file, then starting up Firefox again. I could download and screen capture once more—but only for awhile before running back into the same problems and having to restart my Mac desktop over and over again. It even happens if I don’t use Firefox.

I also did Repair Disk utility—results said nothing was wrong with the hard drive.

Lastly, I tried updating my OS since I had not done so in years. That failed each and every time. Kept getting a popup saying the installer had problems and to re-download it and try again. This happened with every OS version I tried. Even set the system date back to a year that was before the release date for the different OS versions (read about that somewhere)—didn’t help at all.

For reference, I think I purchased this Mac desktop in 2017. It’s currently running High Sierra 10.13 and I can’t install any of the newer OS versions. And for whatever reason my entire system started running slow as molasses about 3 months ago, mostly during boots. Takes literally about an hour and a half after restarting my Mac before I can start using it. I use an external drive for storage of my work, and both it and my internal hard drive have plenty of space available.

One final note: when my desktop started acting up several months ago, I talked to an Apple tech person who walked me through a crapload of things, none of which worked. So I ended up erasing my hard drive and re-installing everything from my last save on my Time Machine drive. That helped make my computer usable again but it still ran slower than it used to.

So is my computer just effed and I gotta buy a new one, or are there solutions that will succeed when all the other solutions have failed? Thanks!