If the crash is happening when you open a new tab, you can try disabling your New Tab Override extension to see if the issue goes away. It may be the cause of the issue.

I don't think this is the answer, as the crash does not happen every time. What does my crash report give as a reason?

the crash report doesn't provide a clear reason as to why the crash is happening. it shows that the unchecky tool running on your system is hooking into the firefox process in an unsupported way though which can cause all sorts of problems.

you could try uninstalling it or changing the following setting in firefox to see if that makes a difference: go to the firefox menu ≡ > options > privacy & security panel and under permissions check the setting to prevent accessibility services from accessing your browser.