FWIW, following one of the recent stable updates and after a period with few major changes in the snapshot inner workings (from what I could tell) I also decided to install stable over snapshot... no problems, but then I deliberately waited for a moment when I thought the versions shouldn't have differences in saved data.
This was a laptop which until a few months ago I most used for work, but it seemed pointless to keep updating the snapshot there as it's rarely used for Vivaldi these days.
Hi, the user data are always in the home directory, /home/.config/vivaldi or /vivaldi-snapshot for example.
.config is a hidden folder like Appdata on Windows.
The browser is installed in the /opt directory like Chrome or other non system browser.
To change the install or user data path it is some Linux knowledge needed, a regular user don´t do this, an advanced user know already how to do this.
So remove 5.6 and install 5.5 should not be a problem, the user data is not touched.
@oelma Create a bookmark in Bookmark Bar
Set as name PiP
Copy the code lines
Paste into URL field of created bookmark
Goto start of URL field
When you are on a page with a video running, when you do not see Picture-in-Picture button, hit the PiP bookmark, you will have Picture in Picture window.