That sounds like Tab Mix Plus is trying to access a window object after its tab has been closed (and the window object force destroyed). Firefox cuts all links to this object when a tab is closed to fix a VERY common memory leak in extensions. Try reporting/see if its already reported in there forum.  http://blog.kylehuey.com/post/21892343371/fixing-the-memory-leak  http://blog.mozilla.org/nnethercote/2012/07/19/firefox-15-plugs-the-add-on-leaks/  http://tmp.garyr.net/forum/viewforum.php?f=2&start=15
I don't know if I didn't recreate it properly before, but the error is now occurring even with TabMix Plus disabled.
Can you please paste the full error message? It could have some hints as to where the error is happening.
It says exactly what the title says it says. "TypeError: can't access dead object." Tried disabling Session Manager as well. Error is still coming up.
It started doing it even when tabs weren't in the background. I rolled back to 14.01, and I am not having the issue anymore.
Similar error since upgrading from SeaMonkey 2.11 to 2.12, using Tab-Extensions Duplicate Tab and Tab Clicking Options (http://twanno.mozdev.org/). It's not happening always when closing a tab, but when it is happening the error pops up every second and you are unable to use the browser, only a restart prevents further error-messages.
TabMix Plus will need to update for the changes.
I disabled TabMix Plus and it still happened, albeit not immediately.
What other add-ons do you have installed? Can you post the contents of about:support?
I did a legitimate add-on isolation, and it looks like it's actually being caused by dragupload. I have disabled it in 15.01 with all other add-ons (including tab mix plus), and am no longer experiencing problems. I will update if otherwise.
Thanks for the update. You might like to inform the author of dragupload. I seem to remember something about TabMix Plus being easily broken by other add-ons -- Kris, do you know about this?
Looks like dragupload is no longer supported (the URL is no longer available). The reviews on the add-ons download portal indicate the problem that I reported here.
It doesn't sound like this is fixed. Maybe wontfix?
I can confirm that after disabling dragupload-extension in SeaMonkey the error does not occur again so far.
Yes, the add-on hasn't been updated since June of last year. I'll mark it incompatible with Firefox 15 and above, to avoid confusion.
TabMix Plus is a common cause of incompatibilities, yes. It modifies a lot of tab APIs (mostly by string substitution and eval), so it's hard to play nice with it, but it's popular enough that most add-ons make special considerations.
I think its negligent to say its a "common cause of incompatibilities" when you do not have data backing that statement up.