Hi,
Welcome to Vivaldi's Forum Tony Hi ยท Smurf Tall.png
First of all,
Extensions support should be added to Vivaldi.
This seems that will not happen, at least in a near future, so, an Extensions Manager is secondary on this scenario.
Also,
Some useful links:
--
Avoid https://vivaldi.com Data loss
Follow the Backup | Reset links below
Vivaldi Backup | Reset + Extra Steps