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?
Go to the extension settings and copy the address
Open Firefox Settings
Select Home page settings
Replace options.html with newtab.html
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