I have the same sluggish issue after my Firefox upgraded to v27. It is unusable now. I have to switch to other browser. Please bring my favorite browser back!! Actually it starts feeling sluggish when upgraded to v26. Also, in v26, I have to kill the Firefox.exe process from Task Manager window every time when I closed Firefox. Other wise, I cannot open another windows. Because it will tell me the same application is in use. In v27, this problem is not as often as in v26, but it still happens. Please fix it! It is unusable and unacceptable.