I notice that Firefox has released an update which sorts out the performance problems they've been having for the last few months.  FF used to be my favorite browser but I switched to Chrome a few months back because of the issues.  It looks like the new version of FF: Quantum, has sorted out the performance problems.  So if you abandoned FF a while back, you might want to give it another go now.  I've been using it for the last few hours and it all seems good now.