B
Weird, I've never experienced this issue. Vivaldi always launches just fine for me from Thunderbird or any other app. Maybe that's because I make it the default browser for all web related settings in the OS, not just the "browser" setting. I set it to also be the handler for the various protocols I expect it to catch, as well as all the file types I want to have open in it. I've got closer to a dozen different browsers installed on my system, including two Firefoxes, so if Thunderbird wanted to open in it, it could. I wonder if it's having configured Vivaldi to handle all the various browser calls that I want, or perhaps it's an install order issue. Thunderbird might check what the handler is for those protocols during its install, and then set its internal options. If you then change it, for example by installing another browser and selecting it as your default, Thunderbird doesn't pick up the change. Could be any number of issues (or possibly more than one), but if I recall correctly Vivaldi was one of the first applications I installed on my current system, so it's always been the default according to any other applications.
