How to install Opera Extensions in Vivaldi browser?
Have you found an extension for Vivaldi browser, that’s not on the Chrome Web Store but available on the Opera add-ons site? Don’t worry! You can still able to install Opera Extensions — which have .nex as file extension – in Vivaldi, here is how. http://techdows.com/2016/10/install-opera-extensions-vivaldi-browser.html
rename to .crx and drag it to the extension window.
Or unpack them with 7zip or alike, then add a not packed extension.