It seems that there is an issue using FF53.0.2 and BitDefender in that FF hangs after one or two windows (tabs) appear to load.
I only installed 53.0.2 this evening having been away for two weeks and struggled to find the source of the issue and tried safe mode, disabling addons, etc.
Eventually a search brought up the following link and, although I use MSE on W7 Pro, I assume I was affected with the same issue as turning off Real-time protection solved things.
An update to the latest MSE definitions and all now seems OK.
Just thought I would post here in case anyone else was having problems.
Forum Moderator and Customer
Courage is resistance to fear, mastery of fear, not absence of fear - Mark Twain
He who feared he would not succeed sat still