That's because that's the version of Firejail that's in the Linux repository of the Linux distro you're using so it's okay to use that version.
You need to learn how to use Synaptic Package Manager
to remove software and install software.
It will do a complete removal of the package and it's dependencies and then after that open the terminal and copy and paste this command.
That will remove any leftovers.
No reason to reinstall your Linux because of installing and uninstalling software.
so much software trying different things and used Synaptic Package Manager
It works great imo and I've only been a Linux user for 3 weeks maybe 4 weeks I don't remember but not that long.
Get on the web and learn about Linux and also the forums has a lot of helpful useful information.
I'm not trying to tell you what to do but I've learned a bunch of Linux stuff just by searching my questions that's the way I learn.
Just my $2.00 dollars worth.