"Your system has run out of application memory" -why? how?

[macstatic]

In the past few days I've had something very strange happen. All of a sudden the "Force quit" window pops up and tells me that "Your system has run out of application memory" while showing that one or several applications have already been paused (whatever that means).
After a little while more applications go into pause. Pressing the "Resume" button only helps for a little while until they pause again.
Here are some screenshots including the "memory" section of Activity monitor (which I'm not sure how to interpret).

My Mac Pro 5.1 (mid-2010) is running OSX 10.9.5 and I haven't made any significant changes except upgrade Firefox and Thunderbird as there were updates to both a couple of days ago. Could Firefox be the culprit with some huge bug gone unnoticed by before release? I'm on Firefox 61.0.1 right now.
I've got 24GB of physical memory so I really don't understand this.

I've been trying to reseach the problem but it's mostly about running server software, some special anti-malware/virus software which I haven't got or High Sierra which I'm not running.
Other than the Firefox upgrade, all I can think of is that I usually never physically power off my Mac Pro but leave it in sleep-mode whenever I take a break or switch it off during the night. Maybe there's some memory leak which I could avoid if I physically turn it off, at least during the night.... Any ideas what this is all about?

 

[Avenged110]

I've had this exact thing happen three times (was super confused at first). I could never figure out what caused it the first time, but the other times it was some kind of memory leak in a single Safari tab. Either way, a simple reboot was the only solution I could find that worked. I also never turn my MacBook off so I'm sure regularly cycling reduces the chances of these things happening, but that isn't going to matter if it's an actual bug in an app. As Mavericks gets older and becomes the last version supported, thorough testing becomes less of a priority, so I wouldn't be surprised if a newer app had a bug that caused an issue like this (and that may be specific to 10.9). I realize that's not very helpful, but for me it's mostly been Safari.

 

[macstatic]

Does Safari also get paused like with me, and do you experience the next app getting paused as well, or just Safari?

 

[Avenged110]

It varies, but this issue usually occurred while I was away from my machine and I only noticed when I came back. As far as I can remember, I've never had it happen while I was actively using the Mac. I've had Safari, Safari and iTunes, as well as all running apps "paused." But in every case, I was busy and had things to do so I just rebooted and it went away. I've yet to have an opportunity to really examine the issue.

 

[macstatic]

I have a feeling turning off the Mac during the night (or for long periods I'm not using it) helps as I've done that the past couple of days and not had the problem return.

 

[Avenged110]

Yea, although it really depends on what it causing the issue. Without knowing, that's probably the only thing you could really do to possibly prevent more occurrences.