Every time I try to upgrade from Technical Preview 4, almost all of my existing plugins disappear. What (probably obvious) thing am I doing wrong? Thanks.
Do you mean plugins, or extensions? They are different things. If you mean plugins, NPAPI Flash and Silverlight are going to disappear, because they are no longer supported. In the future, they will not be supported by any browser.
The plugins for Chromium PDF Viewer stays, but Adobe Reader, Quicktime, MS Office, Google Earth, Java, Windows Live Photos, RealPlayer, Shockwave (as well as Silverlight) all disappear. The plugins for Chromium PDF Viewer stay. The last time I tried a snapshot, as well as the recent Beta, animations and videos all fail and give me a download/external viewer dialog. Frankly, I'm not sure where Vivaldi picked up these plugins in the first place - Opera or IE?
Vivaldi and Chromium do not support these old plugins with NPAPI (Netscape compatible plugins) technology anymore.
Because of security and stability problems.
… Frankly, I'm not sure where Vivaldi picked up these plugins in the first place - Opera or IE?
Some plug-ins are registered with your system's OS and will be picked up automatically if they're compatible with Vivaldi. Some plug-ins may have been placed within folders of certain browsers (Olde Opera worked this way) and don't get picked up. Other plug-ins may simply not be recognized as compatible with Vivaldi, regardless. In other words, YMMV as to what you'll see.
For example, my Vivaldi plug-ins include Shockwave Flash (but only the PPAPI type, even though the NPAPI type is installed onto my system for Firefox's use) and Chrome's pdf viewer. The Nitro and Foxit pdf viewers, MS Office, and RealPlayer plug-ins installed on my system don't appear in Vivaldi.