FF 12.0 saves images of each webpage I visit to my cache folders. Is this a feature or malware?

Question

FF 12.0 is saving images of the top of each webpage I visit into my profile's cache folders.
I'm running Mac OS X 10.6.8 Build 10K549, MacBook Intel Core 2 Duo, 2GHz, 1GB RAM, 160GB HD, and Intego VBX6, updated daily.
In Terminal, the "file" command with the path to an image file, shows e.g.:
.../Cache/7/63/DB1B8d01: PNG image, 201 x 127, 8-bit/color RGBA, non-interlaced
Though the images are small, someone might be able to see, e.g., a credit card number that the user entered!
In the Finder, Quick Look shows each image as the top of each webpage visited, but for externally loaded webpages it is an image of only the webpage portion of the window, not the window's toolbar nor status bar.
Images of webpages are saved two seconds after the page loads (per Quick Look which, unlike Get Info, shows Date Modified time including seconds).
The problem occurs even in Safe Mode after a reboot.
It occurs in FF 12.0 at the administrator user account level and in FF 12.0 running in standard level user accounts.
After backing up my profile and deleting the 04/24/2012 installed update of FF 12.0 (from FF 11), and downloading a new version today 04/29 from http://mozilla.org/firefox (the cache was already cleared from the last FF session), I tested it again, via Help menu, Firefox Help. The page loaded and the cache contained the pages normal icons and images, AND an image of the top portion of this webpage.
The images do not match those used by Tab Groups, nor those used when tabs are moved around each other (comparing the images shows they don't quite match either one).
Keeping the browse window's width the same, but pulling the bottom of the browse window up, until only a couple of inches of the page is visible causes the image to show only the top left corner of the webpage.
If the Add-ons page is also opened and another page is visited (where a new tab is created), an image of the newly visited page is created, AND an image of the Add-ons is created too, but NOT the page the user sees, but rather the older style Firefox window with gray tab buttons in the toolbar showing 4 tabs "Get Add-ons, Extension, Themes, Plugins (which is depressed)". In this window's toolbar on the left are the normal three buttons, each grayed out as if the window is inactive, then the Add-ons title, with the pill button on the right. Below are whatever plugins you have installed with blue lego block icons to the left and their related text, listed below in rows. The top row is highlighted in yellow and that row alone has a "Disable" button. This image even includes a white background around the window. Yet this image does NOT match what the user sees, which is the Add-ons newer full-page style, where the tabs are in a column on the left side and Add-ons are listed in the column on right with a Gear button above, over a blue background.
BTW: I've just pasted the Troubleshooting Information data into the field below, but the copy and paste isn't showing tabs between the preference names and their values, nor return characters, so everything runs together.

Answer 1

I see these, too, running Fx12 on Windows 7. They are described in the (extremely voluminous!) about:cache interface as page thumbnails. I've attached some screen shots.
The text is extremely difficult to see, so I don't know whether it is a serious privacy/security risk. I would be curious to know how long they linger.
I think these support the Windows feature of showing a preview of a window or tab when you float over a taskbar button. (No idea whether there is a similar preview feature on Mac.) This feature is not enabled by default; users need to enable it in about:config. However, add-ons may be able to use the thumbnails because they have privileged access to the cache.
(Note: This feature was added pursuant to [https://bugzilla.mozilla.org/show_bug.cgi?id=497543 497543 – Provide a thumbnail service]. It "landed" on or about Jan. 28, 2012, so its first appearance in a released version of Firefox might have been Fx12.)