tarquin

@burgessmeredith: the way I have done this in the past is:
Rename it from foo.crx to foo.zip
Open the zip file normally. (Normally this works. 7-Zip can apparently cope with the crx files where the system zip utility doesn't understand the crx headers.)
Extract the contents into a new folder on your computer.
In Vivaldi, menu - Tools - Extensions.
Enable Developer mode.
Load unpacked (button at the top).
Select the folder.

With the obvious caveat that if you are using an extension given to you by someone as a .crx, you are trusting them not to be malicious. The extension has not been through the usual moderation it would have had on the extension store.

Note that I mostly do this when analysing or changing the contents of an extension, so I actually want access to the individual files.