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This suggests that the ISO you installed from may have been damaged.
- Did you verify the
SHA256
checksum of the ISO after downloading? - How did you create the bootable USB stick? (We recommend using Ventoy.)
I don’t really know what’s on the minimal ISOs, but to the best of my knowledge, firefox
should already come preinstalled on all ISOs, whether full or minimal.
Either way, after installing, you should definitely make sure that your system is fully up to date. Partial updates — i.e. the scenario where you install newer software from the repos while not updating the rest of your system — are not supported, for obvious reasons.
In addition to that, Manjaro is not exactly the most beginner-friendly distribution, by virtue of its Arch origins. I would therefore also invite you to read the following essay.