The instructions say to download the app image (I downloaded LibreWolf-87.0-1.x86_64.AppImage because the instructions say most modern computers run x86_64), and then says to copy and paste "chmod +x LibreWolf-*.AppImage" into Terminal, and after doing that, I get an error message saying "No such file or directory". I've even tried downloading it again, but changing the file to AptURL when I went to save it (which listed it as an app image file), and it still didn't work. I don't understand what I did wrong. The app image should work on 19.3, right?
First, you don't "install" an appimage; appimages are self-contained, portable executable files. Form https://appimage.org/
Download an application, make it executable, and run! No need to install. No system libraries or system preferences are altered. The key idea of the AppImage format is one app = one file. Every AppImage contains an app and all the files the app needs to run. In other words, each AppImage has no dependencies other than what is included in the targeted base operating system(s).
So, yes, in theory appimages should run regardless of the distro(except Alpine Linux due to different libraries, it uses musl while the rest use glibc, see here
for more on this). I run and use a number of appimages myself, 20+, and they all work just fine. I just downloaded LibreWolf and am writing this using it. Just follow these steps:
1. cd to the dir you downloaded the appimage
2. Make it executable; you can do this in 2 ways:
chmod +x LibreWolf-87.0-1.x86_64.AppImage
via file manager
Open the file manager, locate LibreWolf-87.0-1.x86_64.AppImage, select it, hit Ctrl + Enter
the properties windows should pop up, tick the box where it reads make this file executable
or some similar wording.
3. Run/launch LibreWolf-87.0-1.x86_64.AppImage, again, you have two options here;
hit enter and it should start. Alternatively, open the file manager, double-clikc LibreWolf-87.0-1.x86_64.AppImage or right-click and select launch
or something similar and it should start.
You can make it available in your DE's menu too. In a text editor copy/paste the following
save that as LibreWolf.desktop
in $HOME.local/share/applications. You can also use Cinnamon tool to do that too https://community.linuxmint.com/tutorial/view/1504
I have never been able to get an appimage to work. Believe me I have tried many times.
You're probably doing something wrong.
Also, I think you will need to run the command with administrator or SuperUser privileges.
No, that's not necessary, just imagine that! They can be run as your regular user's permission, no need for sudo and/or root. That would be a security issue and no one, including me, would use appimages.