Sorry in advance for grammar, I’m typing on my phone.
Add-ons can slow Firefox’s initial startup time, but actual load times of pages shouldn’t take any additional time. Have you downloaded any add-ons from sources outside of Firefox/Mozilla?
Also how many tabs do you have open when it crashes? Firefox runs all tabs together which can lead to crashes if you have a lot of them open at once. Luckily it’s an easy fix you simply just need to spread tabs across two open Firefox windows.
The other posible cause is a virus or malware…. Dun, dun, duuuuun!