prefCleaner works fine for me, however updater doesn't seem to be able to append overrides.

Seems to be just a broken check? This fixes it and updater seems to work fine now.
if ! [ "$SKIPOVERRIDE" ]; > if [ "$SKIPOVERRIDE" = false ];

Error in both scripts - you were checking for the literal file named SCRIPT_FILE:

// from
SCRIPT_FILE=$(preadlink "$0") && [ -f SCRIPT_FILE ] || exit 1
// to
SCRIPT_FILE=$(preadlink "$0") && [ -f $SCRIPT_FILE ] || exit 1

and for your missing /bin/sh question - no it doesn't seem to work.

@9ao9ai9ar Thoughts on using #!/usr/bin/env? sh could be busybox, the gimped version of Bash (which is not POSIX-compliant), etc.

@9ao9ai9ar Thoughts on using #!/usr/bin/env? sh could be busybox, the gimped version of Bash (which is not POSIX-compliant), etc.

The POSIX way to do it is to write a program to overwrite the shebang with the path of sh as determined by getconf PATH at install-time (see the relevant section in the POSIX document), but I think that is overkill and I would just ask the users to change it themselves if needed. My personal opinion is /bin/sh is better or at least no worse than /usr/bin/env sh because /usr might not be mounted, and the former allows specifying options to sh as an added bonus. I might reconsider the choice if /usr/bin/env sh somehow turns out to work better for the subset of operating systems the scripts are intended to support, but that doesn't seem likely now that I've verified modern Firefox is not available on Solaris 10.

Hey @9ao9ai9ar @MagicalDrizzle I've noticed that progress on this PR is slowing down, so I'm willing to create a checklist of things to ensure that the script is working properly, and then make a GitHub workflow to test the script on different platforms.

Here are some things to be checked:

• normal execution returns EX_OK
• check every non-zero EX_*
• user.js gets updated
• user-overrides.js is respected

This will tick the GitHub available runners off the list of things that testers have to check manually. Manual tester can then copy the workflow to test locally.

What do you think? An alternative would be splitting this PR into smaller PRs.

@9ao9ai9ar prefsCleaner works, updater only partly - works only if there's already an user.js file. It won't download a new user.js if doesn't exist, then will try to backup the nonexistent user.js file which fails.

+ userjs_backup=/home/demo/Downloads/profile/userjs_backups/user.js.backup.2024-12-01_0511
+ mkdir -p /home/demo/Downloads/profile/userjs_backups
+ cp /home/demo/Downloads/profile/user.js /home/demo/Downloads/profile/userjs_backups/user.js.backup.2024-12-01_0511
cp: cannot stat '/home/demo/Downloads/profile/user.js': No such file or directory
+ userjs_backup=