This is not a new Firefox privacy concern, but I just found out about it. It's existed since about FF12 or so. I wanted to mention it to raise awareness. If you care about your browsing privacy, you may want to take note.

What all Firefox users know is that when you open a new browser tab, FF displays a page with nine screen shots randomly chosen from among your most visited sites. You can toggle the display of the images by clicking a small grey grid icon in the upper right hand corner of the screen. I didn't like the idea that someone looking over my shoulder could see screen shots of the web sites I've visited, so I turned off the new tab display when FF first introduced it.

What many FF users may not be aware of is that toggling off the display does not stop the screen images from being captured and stored in your cache. A new image is captured of the page you are presently viewing whenever you launch a new tab, regardless of whether you've turned off the new tab view.

You can find the images in your home folder under .cache/mozilla/firefox/<yourdefaultfolder>/thumbnails/. I'm not sure what controls the number of png files that are stored there. On my system, there weren't that many, perhaps a couple dozen, but I tend to clear my Firefox browsing history and cache on a fairly routine basis.

Some people may not care that screen shot thumbnails of sites they visit are being recorded and stored by Firefox. In fact, many people may find it useful. But at one time, FF was capturing screen images of https sites, which might include things like your bank login screen. Probably no one would like that. I read somewhere that FF has stopped capturing screen images of secure web sites, but I didn't bother to test it myself to verify. I just wanted to turn off the screen captures entirely.

There is an option you can add to about:config to stop Firefox from capturing screen images for the new tabs page. It doesn't exist by default, so you have to add it. Go to about:config, create a new boolean value called "browser.pagethumbnails.capturing_disabled", and set it to true. I had to restart FF to make it take effect. I've been monitoring for a day, and I haven't seen any new screen shots being saved since I set that option. Remember to delete or shred any existing png files in your firefox thumbnail cache.

What brought all this to my attention was my accidental deletion (via shift+delete) of a folder of important files a couple of days ago. I used photorec to scan the file system for deleted files so I could recover them. Photorec found 50,000 deleted files in total, and among those were roughly 8,900 deleted png files. Of those, I estimate 90% or more were Firefox web site screen shots going back many months, forming a visual history of my browsing activity over that time.

For more complete info, search the web for Firefox new tab thumbnails privacy or similar, and you'll find plenty.

I think this option may do the same thing, because I don't have any thumbnails:
You can try it and see.

Thank you Z038. That is both interesting and useful.
Well done.
jdk

I tried it. It doesn't appear to be related to the full screen image captures that FF displays on a new tab page when the image grid icon in the upper right corner of a blank tab page is toggled on. Whether nglayout.enable_drag_images is toggled on or off, no thumbnail images get written to the thumbnail cache.

What nglayout.enable_drag_images does is control the "drag feedback" image. The description says "Use to eliminate/remove/hide/restore ghost image/picture attached to cursor when dragging link, favicon, picture, or tab."

What I observe is that if it is set to true, a copy of the object being dragged on the UI will be attached to the cursor while dragging. If set to false, a generic icon appears instead of a copy of the object. The difference is seen easily on any LQ page by toggling the setting on or off, and then dragging a link, or the group of four Tux's that appear in the upper left corner of the screen, or an icon, etc.

Most of the objects that can be dragged are already discrete elements of the page, such as a gif, png, or jpg, so perhaps FF doesn't have to make a thumbnail of them. A copy of a link being dragged may be a different story. I don't have a clue how a GUI or windowing system works. If there is a cache of dragged objects or images of them being stored somewhere, it could also have privacy implications, but if such a thing exists, it doesn't appear to be in the thumbnail cache.

You're welcome jdk. I would probably never have noticed the cache of screen images if I hadn't run photorec to find deleted files in the free space of my drive. It makes me wonder how space is allocated and reused in an ext4 file system. That photorec was able to recover over 8,000 of these png images created by FF going back at least a year was quite surprising to me, given how much churn I have in the file system mounted on /home.