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Firefox uninstalled itself :|
(feddit.org)
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0
More than likely it was a failed package transition that failed. You were running one version, an update triggered, something went wrong, and your data folders got orphaned. You can try running a repair on the package, but they usually fail the same way.
Anytime there is an update, files are often deleted during that process so they can be replaced with new files or because those files are no longer part of the new version being installed. If an error occurs during this process, it is possible that an application will appear not to be installed because it's broken.
Anyway, most software does at least partially "uninstall" when it is updating, so if the install fails, then it's always possible that an update will have uninstalled something. That's just updates regardless of operating systems, package managers, etc.