Vivaldi + Thunderbird issue
This is slightly complicated, so I'll explain as best I can. I opened a Vivaldi window on my primary monitor that I use for most things. I opened a second Vivaldi window on the second monitor just for Facebook. I also have Thunderbird open on the second monitor. If I click a link in Thunderbird, it opens as a new tab in the last active Vivaldi window. But then regardless of which window that was, the second Vivaldi window pops up and takes focus. Even if I set the Thunderbird window to always on top to prevent Vivaldi from obscuring it, the focus switch still happens, so if I switch from the first Vivaldi window to Thunderbird and then open a bunch of links, the first loads in the first Vivaldi window but each of the subsequent ones loads in the second Vivaldi window. I think this just started with the latest update, but I can't be sure. This is version 1.4.589.29 (Stable channel) (64-bit).
i can use thunderbird fine. which linux version are you running? keep in mind there are multiple threads for vivaldi windows open on more than one monitor.