Disable clipboard events

To prevent sites interfering with cut-paste:

dom.event.clipboardevents.enabled = false

Hide Tab Bar

~/.mozilla/firefox/$profile/chrome/userChrome.css:

#TabsToolbar { visibility: collapse !important; }

Enable in about:config

toolkit.legacyUserProfileCustomizations.stylesheets = true

Fix "Already Running and not Responding"

Shut down firefox and remove the lock files in ~/.mozilla/firefox:

rm ~/.mozilla/firefox/*/.parentlock ~/.mozilla/firefox/*/lock

Prevent websites overriding shortcuts

In about:config:

permissions.default.shortcuts = 2