To the second point, as a avid firefox user, I noticed that some Webapps seem to not depend on the Browser alone.

Even in safe mode, some Webapps sometimes work better on different systems than on others using the same Firefox version.

For instance youtube streaming seem to work better on my Linux laptop then on my Windows desktop, where it becomes stuttery. In Chromium there it works as well as Firefox on my Laptop.

What I want to say is that browsers and all the systems around this are very complicated. So your milage with the same browser will vary, and you might blame the wrong thing.