Firefox (like other browsers) has its own internal proxy settings. CLI applications such as apt-get don't see this, they rely on an http_proxy (yes, lowercase, AFAIK) environment variable:
I've found the source of the problem.
There is also a file for environment variables:
where https_proxy had =https://
Changing it to http:// and then restarting
sudo systemctl restart snapd.service
fixed the problem.
Proxifier for linux:
As of 2019 there is no bundled application that does all what proxifier does instead you can use different applications for each proxifier feature.
Block network for an application
Here is a detailed list of solutions for this purpose
Route by ip
Linux routing table can be used to add rules to the routing table and thus route ip/ip-...