SOCKS isolation should include a process identifier.
This isn't an issue when using Tor Browser with "tor-launcher forks/execs the tor process" model, but is relevant to all other use cases, particularly with a system tor instance.
The SOCKS username/password isolation should include a instance identifier such that each invocation of Tor Browser ends up using difference circuits (Currently, the isolation tags will get reused).
The current format is
domain:counter. The naive implementation of this would be something like
pid could be expanded to include process launch time information or similar to handle the PID reuse case, but I'm not sure how likely that is (the entire PID space needs to be consumed before PIDs are reused on Linux).
I filed #18125 (moved) a while ago with similar rationale in mind, but doing it this way is better, so feel free to kill off the older ticket in favor of this one.