This would be amazing. The main reason I've never used Tor is the fear that it would make me look like I had something to hide (instead of just a general desire for privacy). If it were built into Firefox, I'd probably switch over from Chrome.
1. You can hide the fact that you're using Tor by using pluggable transports which are already built-in the Tor Browser (such as meek-azure, obfs4, snowflake, ...).
2. That's the biggest reason as to why one must use Tor as much as possible even if they don't care about privacy. More people using Tor = the less interesting it is to be a Tor user.
That's exactly why I use it on a regular basis!
Perhaps when looking up political or medical things (also for friends), it could be good to run that through Tor. That is not something to hide per se, but it is definitely not something that is anyone else's business, and you don't want to be bubbled.