In my workflow I often need to open a lot of tabs simultaneously. Sometimes 80, sometimes 300. So I'm not surprised when it takes tens of gigabytes of RAM (though previously, before Firefox Quantum, 6Gb was perfectly enough for all the same pages in same quatity, nothing has changed in those pages' code), I have plenty of RAM. What has drastically changed is disk space usage... Firefox EASILY eats out 30-40 Gb on SSD, sometimes even crashes as it eats out all the free space. It's not windows file cache, it's not modified cache setting in about:config, there is still more than 20 Gb of free RAM... Yet it keeps writing TENS OF GIGABYTES of unknown stuff on SSD, effectively killing it. What's going on? Am I the only one who actually need 300 tabs simultaneously so nobody else notices this problem?
I noticed today when using Photoshop that I couldn't re-size an image due to low space. My SSD was deep in the red, as you say, 40gb of data was put there. Somewhere, for some reason. I also would like to know why this is, because it is a bit weird. 22Gb of RAM, okay, no problem. And I would expect maybe 20mb of data in terms of cache or whatever, but what is possibly 40GB of data doing on my SSD ??
Sorry this is not an answer, but at least you know you're not alone with this concern/issue.
Also, OP, I am curious but it is of no consequence - mind if I ask what you do to require 300 tabs? I honestly can't guess !
Some folders that can grow substantially as you browse:
(1) Web content cache
You can use the about:cache page to check the size of the memory and disk caches. The page also lists the path on disk to the cache folder.
(2) Offline website storage
You can use the "Manage Data" button on the Settings page, Privacy & Security panel, Cookies and Site Data section, to see which sites have stored a lot of data.
If you use Mega for downloads, some databases may get swollen. I don't know whether removing site data will fix that or whether you might need to go out to disk to remove some files.
(3) Add-on storage
Extensions don't normally store huge volumes of data. It can be difficult to determine what exactly they are storing, however, so I would check the others first.
Cache never shows more than 100 mb.
Maximum disk storage is set to 1Gb and rarely goes above 300 Mb.
Yet it keeps writing something in tens of gigabytes every time. Though I noticed it doesn't happen linearly with gradual use of space. Like I look at the disk and it has 40 Gb of free space. Then in 10 minutes I look at it again - same. Then I continue with my work, download something, open and close few dozens of pages. Then I look again and bam! now only 12 or 10 Gb is free. It must be something else.
Btw files I download never amount to more than 2 Gb in total and download folder is set to different bigger disk.
I do use few extensions to automate the procces. Namely Snap Links to open all the pages that were updated with just a couple of clicks. Also I use quite popular DownThemAll addon to parse the links matching a regular expression on all the updated pages and downloading them. This same addon I use in Opera browser and tested it for control. It doesn't show such behavior there.
I should probably add that when I close all Firefox windows (or when it crashes due to lack of space) the disk goes back to normal state. All the space becomes free again as it was before without me manually deleting anything or running the process of deleting the temp files.
The space is released in a crash? When Firefox crashes it isn't able to clean up its own temporary files. Could the disk usage be in the swap file -- in other words, extreme memory usage that needs to be paged out to disk?
Does the about:processes page show any unusual memory usage patterns?
Yes, exactly. Space release also happens if I terminate the firefox process from the task manager.
But no, that doesn't seem to be swap. As I said earlier I usually have at least 20 Gb of RAM free (out of 64Gb total on board) when excessive usage of disk space happens.
Though I'll try to monitor memory usage by Firefox more closely.
Turns out it was Windows Hibernation function working incorrectly with Firefox browser. I found out hiberfil.sys file grew immensely (going above 80 Gb at times) while using Firefox in the scenario described above. Strangely though I tested several other browsers (including popular ones like Chrome, Opera, Edge and Vivaldi) and it never was as critical. Disabling the hibernation solved the problem for good. Not a single crash since that moment. So I can recommend turning it off on any machine with more than 32 Gb of RAM.