I would like to use KEY in private browsing of Firefox.
LastPass works in private mode and also some other add-ons I use (e.g. Pushbullet). Also, KEY works in incognito mode on Chrome.
Sorry for my potential feedback (I'm also only F-Secure user).
I'm not sure how it was previously, but currently with up-to-date Firefox and F-Secure KEY (and their extension for Firefox: it works with private mode browsing too (and?! with default state of settings).