How to Restore Firefox to Its Default Settings and Start Fresh

Any browser can slow down and become cluttered over time as you install add-ons, build up history, and change settings. Firefox can “Refresh” your profile to quickly give you a clean slate while keeping your most important data. This would let you try the overhauled Firefox Quantum with a fresh profile, speed Firefox up if it’s become slow, or fix other browser problems.

via: howtogeek.com