It used to be that Firefox (profiles) can only be reset while using the Safe Mode, (2), (3), but as of recently the developer test versions have a about:support page with a button to allow "Reset Nightly to its default state". The about:about page has a list of all available "about:" pages in Firefox, also. PS. Btw., the command launching Firefox in Safe Mode is:
firefox -safe-mode
PPS. Safe Mode will also allow a selective-startup, where the component causing the hang can be easily identified. :f