I was having similar slowdown problems with FF (29.0.1) recently - never-ending rotating blue circles (aka hourglass) when loading webpages and content. It always started reasonably quickly but was horribly slow in doing things.
Did many FF resets which is a suggested remedy, and seemed to help temporarily. At one stage I suspected it was the Foxit pdf reader application but after removing this it eventually didn't improve. It was almost like the machine had forgotten or lost its RAM except all the applications outside FF worked perfectly and at top speed.
The solution for me was remove Avast AV and replace with AVG. It may have been that Avast was having its own little problems but nonetheless with it now running as fast as Formula 1 with AVG I didn't want to tempt fate.