@funtudeka,despite Vivaldi is only 95% FOSS, the script is 100% auditable, there isn't any problem respect security or privacy.
I personally also like to use FOSS as much as possible, but I also know that FOSS is not always the best option. The point of FOSS is to facilitate collaboration and development of new software, but this in case of browsers, in view of already around out of 100 browsers that exist, it has a debatable value and they reward the manufacturer's ethics more, and this in this cooperative owned by the employees, focused on the needs of the user, which is Vivaldi, is fully guaranteed.
The security and privacy of a software depends always of the author's intentions, independent if it is FOSS or not, in fact, all the APIs of Google, Facebook, M$, Amazon, etc, to track the users, are also FOSS and included in many FOSS found on GitHub, which in turn is owned by Microsoft.
Adding FOSS by definition it is only as safe, if it is actively maintained with an active community, nothing more dangerous than an unattended or abandoned FOSS, as precisely because the script is public, it is easy prey to manipulations by malicious people, who do not even need to bother using reverse engineering to decompile it, as already shown in several cases.
You can therefore be calm and enjoy the, not full FOSS, Vivaldi.
Welcome
