What is the version of your Firefox. Your OS? Maybe it happens because you have a slow PC? or laptop?

Thanks for following up

FF = 61.0.2


16 gigs of RAM i7-7700

Happened with Win7 on a Core2 Duo as well

I am a member of Nimbus :).

Can we get your backup? 

It can happen because of number of dials and groups.

Did you try this?

1 Go to the extension settings and copy the address http://nimb.ws/H1W7qP

2 Open Firefox Settings http://nimb.ws/cokXtr

3 Select Home page settings

Replace options.html with newtab.html http://nimb.ws/CMxYdg