Only allowing to complete the survey if you use certain browsers on mobile
It asked me which browsers I use on mobile and didn't let me complete the survey unless I answered that I use one of the browsers in the list (which I don't do). Because I already spent time on the survey, I incorrectly had to reply that I don't use a smartphone so I could complete the survey.
You'll either lose all the survey results from people who don't use one of the browsers you considered, or have them giving wrong information at other places.
I use the default browser on SailfishOS by the way.
Only permitting to having used Firefox since recently
In the survery, I said that I use Firefox since more than 3 months. Still, the survey asked me whether I just started using Firefox recently, started using Firefox recently after a break or used Firefox consequently since recently (whatever that was supposed to mean). Maybe this is just an error in the German translation.