Vivaldi crashing on every occassion of closing Thunderbird.
Each time I close Thunderbird, Vivaldi would crash with loosing last session. It is one of many weird things Vivaldi does. It is the least stable browser I have ever worked with! But I love it and would like to see some stability improvements! The session management need to improve too - if I open new window and the accidentally close the first one, or it crashes, the whole entire session is lost and new Vivaldi instance will open just blank. I have lost hours of work because this bad behaviour.
Never happened to me, I open/close thunderbird many times a day, and I even find strange that the 2 programs could interfere with each other.
Cant confirm this with Vivaldi 1.10 (64bit) and Thunderbird 52.1.1 (32Bit) on Windwos 10x64.
Like @iAN-Coog, I've never had this happen when closing Thunderbird on my Win7-64 system, and I've been using Vivaldi from the beginning. Given that the architecture of Thunderbird is totally different from Vivaldi, it's hard to see where the areas of confusion/interference/overlap might lie for the OS, though perhaps there might be some potential areas in default file-opening settings on your system.