I understand your intention is to launch "cold" Firefox instance that does not call home, update the "safe browsing" databases etc, so this will probably not be of any help, but you may try launching it with `--offline` command line argument and uncheck the (alt) "Menu > File > Work Offline" when you are ready to "reveal yourself". For testing it with throwaway profile you can do:
firefox --offline --no-remote --profile <path where throwaway profile folder can be created>
The `--offline` argument is quite strange for a several reasons:
1. it is quite undocumented, but works from what I've tried. It is not even exposed in `firefox --help | more`. It used to be mentioned at MDN page [1] but that is gone now.
2. From what I've tried now, offline mode lets you browse localhost, what seems super useful but I don't recall it have always been this way.
3. Also browsing properly cached pages seems to work super well; not sure it is due service workers finally caught up and works or proper HTTP headers are used nowadays. Interesting is that even the hard refresh of cached page gives you cached version in offline mode. (Again, I'm not sure it used to be this way in the past. I remember that nearly no webpage worked well in offline mode few years back.)
I have not run any network traffic audit so I cannot verify it really does not attempt to reach for something outside localhost.
[1] http://web.archive.org/web/20210530092017/https://developer.... (I have a super murky memory it might be me who added it there, but today I had to find SU answer mentioning it [2] to even try that)
[2] https://superuser.com/questions/1691419/how-to-start-firefox...
(Update) And one more thing: when using Profile Manager there is a nifty "Work offline" checkbox in there. Run
firefox --profilemanager
There is also checkbox "Use selected profile without asking" that when unchecked presumably adds
[General]
StartWithLastProfile=0
to the `<appdata>\Mozilla\Firefox\installs.ini`. I've found this handy to do if you occasionally mess with various Fx versions and want to be sure you will not unintentionally load "nightly" profile with "stable" executable.