Yes, this is a bit concerning as Vivaldi development was last I heard self funded by Jon von Tetzchner (http://www.wionews.com/science-tech/wionspecial-ceo-explains...). He claims it's otherwise meant to get funds from search engine and bookmark bundle deals. Sounds like a fragile base to build on in a niche browser aimed for power users, who tend to be unusually quick at removing/reverting exactly such features?

Isn't the way out of this via open source and community efforts essentially why Firefox has been successful despite not being OS bundled or backed by a huge search engine company like Google?

No, Firefox is successful because it's lineage from Netscape, and then subsequently releasing its browser during a time when the only competition was terrible. If Firefox had started today it would have the same problems as Vivaldi.

Firefox definitely does benefit from community contributions (a lot), but the vast majority of the important code is written by paid employees.

It seems to me that you're conflating "open source" with "developed for free".

I don't think GP has any issues with Vivaldi being developed by paid developers per se, but if Vivaldi Technologies decides to stop development there is no guarantee that anyone else will be able to take over development. Firefox wouldn't have that issue if it started today.

I'm not conflating anything.

I don't disagree that it would be better if Vivaldi were open source. I just don't think that reason is all that compelling to Vivaldi the company. Who thinks about "what happens when my company folds"? There are other advantages to being open source than just that.

I think, in this case, it would seem the company founder(s?) might be likely to care, given their apparent reasons for starting the company. Jon left Opera when it started to go south in terms of serving it's existing user-base, and very clearly started Vivaldi explicitly to serve that orphaned user-base. It does seem like he would/should care if history repeats itself with Vivaldi the company.