focus on lower end devices and that's not inherently stupid.
It is. Phones are an aspirational market, it's the top end that sets market trends. It's been the case since 2007 at the very least, and arguably well before that. Focusing on the low end was a huge mistake from Mozilla leadership, and it's sad that nobody seems to have paid a price for it (beyond the FFOS team, which was eventually disbanded). FFOS almost killed Mozilla.
Jerboa is pretty bad in many, many ways; at one point it almost killed Lemmy for me. Sync is at least polished and robust. I've not seen ads yet but I wouldn't be opposed to paying something for a decent client - I used to pay for Narwhal too.