Firefox 15 introduced a major improvement in memory usage, by disallowing add-ons to keep references to DOM objects after their parent document was destroyed. This eliminates the most common cause of memory leaks in add-ons, and should reduce memory consumption for many users.
If you install an add-on that causes this kind of memory leak, starting with Firefox 15 you’ll see errors like this in the Error Console:
Error: TypeError: can't access dead object Source File: chrome://addon/content/file.js Line: 42
This means that Firefox did the right thing and prevented the add-on from holding on to this object. However, it also means that the add-on probably won’t work correctly.
If you run into this error, please contact the developers and point them to this post. Or get in touch with us on the comments and we’ll try to contact them ourselves.