firefox offers older offline installers on an ftp and getting an older offline installer for chrome isn't hard as for automatic updates
for firefox you can create a json that turns off updates or remove some installation files
for chrome you set "gupdate" to disabled & set "gupdatem" to manual in services
2021 edit
in at least Chrome 79 setting the clock to before January 12th enables Flash Player date can be changed back once loaded in
this isn't needed at all in Firefox 46 i'm guessing somewhere between Chrome 59-68 don't need it either