Re: The problem
I run Firefox 28 on one laptop and the latest Firefox 42 on another laptop. When I compare the two, browsing identical websites with multiple tabs open, not only is FF28 faster but it uses about half as much RAM. FF42 gobbles up RAM at an alarming rate, when FF28 is using 750M RAM, FF42 is using 1.3G and it gets worse and worse as you keep browsing.
It's almost like the modern versions of FF act like old Windows 98, using all all available RAM and getting slower and slower until you have to restart it to get it running fast again.
FF jumped the shark with the Australis interface. There's a good reason why the classic theme restorer plugin is popular.