I'm one of them. I switched to Brave first, and have settled on Vivaldi going forward. Why? Because Firefox was increasingly problematic to use. Things like background blur on Google Meet didn't work.
Yes, I realize it was probably Google doing Google things to hurt Firefox, but day-to-day, I need a browser that works. So switching to a Chromium-based browser that had things like ad-block and other features I consider important was the best solution.
I have worked at Google (although in a different department). From what I could see there was never intentional hurting of other browsers. However, given a list of (always too long) TODOs, improving a non-critical feature for 10% of users often just doesn't make it to the top.
And yes, Google is a huge company with a ton of developers, but teams are not. In my experience, these kind of fixes are sometimes even done by individuals in their "spare" time, to contribute a bit back (even if it's for a competing browser).
Yes, I realize it was probably Google doing Google things to hurt Firefox, but day-to-day, I need a browser that works. So switching to a Chromium-based browser that had things like ad-block and other features I consider important was the best solution.