Firstly it felt annoying because it popped up when I opened firefox (not for the first time today) and had a 'Start browsing button' which immediately drew my eyes, but then I noticed the two checkboxes that were checked by default: 'make default' and 'pin to taskbar'.
That feels deceptive, which makes firefox feel deceptive. 'Start browsing' shouldn't apply changes to my PC. I feel like I have to be on guard using firefox now lest I get something I don't want by mistake, and that makes my browsing experience more stressful, which consciously or subconsciously reflects on my opinion of firefox. I would suggest one of these changes to make it feel better:
1. While the options are checked, change the button text to 'Apply changes and start browsing'
2. Add a previous 'page' that says 'You've been using Firefox for a while now, we have some settings for you to customise' with a 'Next' button before the checkboxes appear (and ideally a 'Skip' button).
3. Have the checkboxes unticked by default, so that it is Opt-in, instead of Opt-out.
