When does Firefox crash?
What are you doing at the time of the crash?

It mostly crashes after leaving firefox open but idle for a limited amount of time (let's say 30-60 min) and restarting to browse on the internet (nothing special).

Please try Firefox in Safe Mode. ^{More Details}

This will allow us to rule out the possibility of this crashing been caused by an extension/plugin on Firefox.

Please report back soon.

I'm going to try immediately. Only it will take time before I know it is stable (because crashes are unpredictable). Thanks so far.

Hi Wesley, in safe mode no crashes so far.

Ok.
This issue is most likely caused by a Firefox extension.
Further instructions can be found here.

Please report back when your problem is solved.