These two reports are "Out of Memory" crashes. This occurs when Firefox requests a block of memory from the system and the system declines the request. The reports show memory is 93-94% full in the two cases, which is pretty high. What I can't tell is whether Firefox is using up the memory, or whether so many other programs are using memory that Firefox just can't get enough...

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Thank you so much for your replies. Indeed, I have many tabs and instances of Firefox open. Though in the past all this has caused is a slowdown of my system and not an automatic closure of Firefox without any prior prompts.

I realise now that I was also running Chrome at the time. After closing it, so far so good with no crashing....