700 MB isn't exceptionally high memory usage for Firefox.

How much physical memory do you have installed on your computer ?

Your Mac OS should also be able to swap unused memory to the hard drive for extra virtual memory. I don't know how this works on Mac, Windows has a page file and Linux has a special swap partition and I assume that Mac functions the same as Linux.

Thanks for the reply. I agree it isn't a lot and I have 16 Gb of memory, and strangely my activity monitor never shows any pressure at the time and shows 4-6 Gb of free memory, but I usually have the message after not using the computer for a while, so I don't actually see it pop up, but it's there when I go give password and start working again. It does seem like a lot of memory for 1 window with 1 tab in FireFox, and as I said, no messages since quitting use of Firefox. I'll post if it comes back when Firefox not running and start bumping up the number of windows I run in Chrome to see if I can get any change in response. At first I suspected something like Malwarebytes running in the background. Yes, the Mac will use the hard drive for extra virtual memory and I checked to see if it was full, but only about half used. So far Firefox is a likely suspect. I guess I could quit Chrome and only run Firefox and see what happens.

Is this message coming from the Mac OS or from Firefox as the latter might have other causes like exceeding your DOM storage quota ?

What is the size of the profile folder in both locations ?

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• ~/Library/Application Support/Firefox/Profiles/<profile>/

Location used for the disk cache and other temporary files (Local Directory on about:profiles).

• ~/Library/Caches/Firefox/Profiles/<profile>/

It's from the OS. I didn't find any Firefox directory in either of those paths.

Thanks for the trouble shooting. I think I'll try my plan to run just Firefox and see if I still get the problem.

Note that in Mac OS X v10.7 and later, the "~/Library" folder in the Home directory is a hidden folder.

• https://www.macworld.co.uk/how-to/mac-software/hidden-files-mac-3520878/

The Application support one is 538 Mb The Cache one is 398 Mb

I already had the hidden folders as viewable, I just went down to root to try to follow the path.

You can find a button on the about:profiles page to go to each profile folder.