I just had the same problem; See here for two possible solutions:

https://support.mozilla.org/questions/997126

I definitely don't like this new format. It makes our machine run so much slower. It is also very cumbersome to use. For a group that is supposed to be consumer oriented you sure make it difficult for us to customize our settings. Please bring back some more of the customize settings...at the very least fix it so we can set our tabs where we want them, not you.

I agree!