I am a creator of a web-based image editor www.Photopea.com and here are things I miss in Firefox (and why my users prefer to use Chrome):
- impossible to put an image into a clipboard (so that user can e.g. paste it outside a browser)
- no PWA mode (impossible to put an icon of a website to your homescreen, which will open the website without a browser UI)
- impossible to have keyboard shortcuts with Alt https://bugzilla.mozilla.org/show_bug.cgi?id=1568130
The thing I miss most in Firefox is the Smart Bookmarks feature they removed a while ago. Smart Bookmarks were fantastic. Add your favorite sites' RSS feeds to your bookmark toolbar and you'd have all the recent headlines from all your favorite sites at one click. Fortunately I wasn't the only one that appreciated this long neglected feature so someone created Livemarks (https://github.com/nt1m/livemarks/) that mostly replicated its functionality, but it's not quite the same as having native support for them.