Well the purpose of a Secure browser is not too all all and sundry Add-ons to be added, so there will be a limited range of approved add-ons.
If you us it only for secure tasks that shouldn't be an issue, but by all accounts it does allow some. I don't use ASB so I can't speak from personal experience..
I suggest you read this FAQ -
https://support.avast.com/en-us/article/29/#pcParticularly scroll down to - What is Extension Guard? - and - Why does Avast Secure Browser block some downloads?
Hopefully that will give you an idea.