I like Vivaldi and they are going to keep V2 support for a while. I will switch to Firefox when it's gone, but for the time being I am happy they are keeping the support.
I would totally use Firefox if they had better first-party tab group support and syncing workspaces. They help with my tab messes which I need to keep organized
And even if they don't keep it: they got browser-level Adblock- and Tracking-Filters that you can just feed the same lists you'd put into uBlock
Sure it's lacking the spot-blocking, tool if there's a missed ad or a fine-tuned whitelisting but I think that browser will stay usable even if V3 is implemented.