Sounds like you have the pref to disable setting src on an image set (look for capability.policy prefs in your prefs.js).
The following is set in C:\Program Files\mozilla.org\Mozilla\defaults\pref\all.js pref("dom.disable_image_src_set", false); No other .js file seems to have a relevant preference set.
Do you see the same problem if you create a new profile and run with that profile?
WORKSFORME, no feedback from reporter - Reporter, if you see this problem in a recent build of Mozilla, feel free to return and reopen the bug. Marking WORKSFORME