Firefox Nightly is experimental and sometimes it can be unstable. With that being said…
Does the issue occur with the Stable build?
Does the issue occur with a fresh profile with no addons?
Have you tried via about:support (Troubleshooting Information), click Troubleshoot Mode and test if the issue occurs?
Is your system low on RAM? Do you have swap enabled?
It would be helpful if you posted some system information:
inxi -zv8
Most Firefox issues are caused by corrupt profiles or addons that are misbehaving.