extensions: - TreeStyleTab (0.14.2013100100a101917) - Ctrl-Tab (0.21.1) One of them is probably the cause, I suspect TreeStyleTab since I'm using Ctrl-Tab on other machines and I don't have any issues IIRC. Symptoms: after some time of using a single window with many tabs ctrl-tab stops switching tabs and the error appears in browser console I so far was not able to discover what triggers the error. New windows work well, reopening the badly behaving window helps too.
You *are* using an addon. This isn't a core feature of Firefox.
Please report this to TreeStyleTab's author(s).