Why would anybody insist on using them? Isn't there enough of 3-rd party cross-browser bookmark/passwords/whatever syncing tools available?
Edit: just to clarify, I have an iPhone, and I run Linux on my laptop. Whenever I tried using Firefox on my phone, I had a bunch of problems with it, so it is quite limited what I can use to sync between my two devices.
FF is great but the "send to device" feature has not worked for me in years. I suspect it got confused when I migrated my Apple profile to a new phone.