My question is about Firefox and Chrome.
Is there a possibility to see which sites have set the HSTS flag in my browser?
chrome://net-internals/#hstsin the address bar of chrome
%APPDATA%\Mozilla\Firefox\Profiles\into the address bar of the file explorer (for Linux it is
~/.mozilla/firefox; for Mac, it is
SiteSecurityServiceState.txt. This textfile contains sites that have enabled HSTS.
There is a Firefox plug-in called PinPatrol that lists all sites (preloaded and visited) known to have HSTS support.