Ranked in these QuestionsQuestion Ranking
Pro Access to new features
Nightly comes after beta which means you get access to more features.
Pro Blazing fast
Short load times. Quick tab switching. Fast rendering.
Con Buggy; performance issues and crashes
Because you're accessing new Firefox features, bugs and crashing comes. Firefox Nightly is meant for developers and it's for PCs that have low chance of getting attacked. If Firefox is for you, go for the normal one or beta at least. If you use Nightly, checking updates regularly is recommended.
Con Not intended for general use
This browser was made for testing purposes for future versions of Firefox, and it can easily crash. Most people should use the stable release of Firefox.