This seems to be a Firefox bug, which I reported now as
bug 908069.
There's a workaround to fix this described in
issue 5746. All you have to do is to remove the entry for SVG files from your
mimeTypes.rdf file inside your profile folder.
If you want to go back to 1.11.4, you can
download it from addons.mozilla.org. Though you'll have to disable automatic updates inside the Addons Manager, otherwise it is automatically updated to 1.12 again. Though note that you won't get any updates for your add-ons then and also won't get to know when it's fixed. So I suggest you try the fix related to the
mimeTypes.rdf I mentioned above instead.
Sebastian