Bug 376997 added a stylesheet to top-level images that forces the background to black and centers the image. This extension removes the stylesheet, returning the background to your configured default and removing the centering.
There are also options for setting a custom background color or a checkerboard background, and for re-centering the image and removing the margin, so if you prefer some different combination of styles you can set that too.
Version 3.0.4 hangs if there are multiple spaces in a row in the image page title (from the filename, or with Greek and Luganda locales.) 3.0.5 fixes the hangs.
3.0.6: Don't hang if the image filename begins with a space.
3.0.5: Fix for hangs with Greek/Luganda locales, or when displaying an image with consecutive spaces in the filename.
3.0.5: Fix for Firefox 26 compatibility (bug 677912).
3.0.4: Proper fix for hangs.
3.0.4: Make the Help button work in Seamonkey.
3.0.4: Workaround for images in frames being resized to 0x0px.
3.0.3: Work around hangs on non-English locales.
Override Firefox's built-in image resizing. This fixes the issues with images close to the viewport size not resizing when the page has margins.
Add option to change the background of the image itself separately from the page (useful for transparent images on dark backgrounds, or to make the background a checkerboard). Defaults to "white" unless you're upgrading from a previous version and had the "Checkerboard background" option turned on, in which case it defaults to transparent.
Support using the string "checkerboard" in page/image backgrounds to get a checkerboard background.
Some add-ons ask for permission to perform certain functions. Since you’re in control of your Thunderbird, the choice to grant or deny these requests is yours.
Please note this add-on uses legacy technology, which gives it access to all browser functions and data without requesting your permission.