How was the flame graph created? (Not very familiar with C and the performance tools around it)
It will spin up a localhost server after the trace ends, the profiler uses the localhost server and nothing is shared with Firefox servers unless you explicitly choose to upload the data and create a permalink.
You record performance data with `perf`, then use the scripts there to turn it into a SVG.
https://github.com/google/pprof
go install github.com/google/pprof@latest
pprof -http=: prof.out
I normally collect the profiles with gperftools (https://github.com/gperftools/gperftools) and then just LD_PRELOAD=/usr/lib/libtcmalloc_and_profiler.so CPUPROFILE=prof.out <your command>
I've been meaning to try Samply though. Not sure if it works with pprof.