To be fair, this is (relatively, compared to the age of the web) new behavior though.
Even Chrome, which pioneered the "tab per process" model, didn't isolate same browsing group context sites for a long time, and still shares processes on Android as a resource optimization: https://chromium.googlesource.com/chromium/src/+/main/docs/p...
Firefox only released their "Project Fission" in 2021, i.e. three years after Spectre.
