On one of my Firefox installations, when I start it, it always asks me to become the default browser. Which I don't want, so I set the "don't ask me again" checkbox. Yet it will ask me again. The Firefox support forum mentions several workarounds, none of them working.
Also, there are additional random /modals every once in a startup, asking me to try out new design feature X. I'm never interested, I just want to use a web browser.
All in all, I'm more annoyed by Firefox than other browsers.