There is no modem with FiOS.
However, you do need their router, at least for activation of service, and if you have TV service, it's just simpler to use their router.
Though it IS technically possible to use whatever router you want if you feel like going through the effort, there are guides online.
TV quality is above comcast's generally. While there are no 4k offerings currently on Verizon, their 1080p content tends to be the highest quality available among the big cable companies.
I personally haven't used their TV service in several years, but I have had 15/5mbps, 25/25mbps, 75/75mbps, 150/150mbps, and now 1gbps (940/940mbps) from FiOS continuously since about 2008. With the 1gbps service starting April of last year. I can say their internet service is nearly flawless, there have been some occasional rare late night DNS issues, though they're usually short term events isolated to late night hours. The biggest complaint I have is with their peering to youtube. During evening hours, youtube simply buffers, even at 720p and 480p res. With 1gbps internet, you'd think a 5-10mbps youtube video would have no issues. At the same time this is happening i can download a torrent at 500mbps+, or steam games at 600-700mbps+. But youtube still buffers due to saturated peering points. You can get around this with a VPN, but it's inconvenient.
Besides that though, FiOS is by far the best internet available.
Though I will say comcast does offer their 2gbps fiber service here, tempting, but still a bit pricey for me considering the local network upgrades i'd need to do to take advantage of that.