My boss intercepts all traffic in the corporate network, including HTTPS (by making computers trust the company certificate). Is it still safe to use Tor in those conditions?
Define safe. If you're worried about your boss being able to see what you're doing in Tor, then yes, Tor will be safe.
Your Tor traffic will still be intercepted by your boss, but since the connection from you to the Tor network is triple-encrypted, your boss will only see a bunch of encrypted data going from you to whichever guard relay you're connected to.
The only problem you will have is a pluggable transports : you'll probably encounter poor TLS/SSL MitM attempts and you should try Meek and OBFS 3 and 4 : sometimes in HTTPS sniffing cases OBFS v3 will fail but OBFSv4 will succeed, sometimes vice versa. If it still fails - try Meek. Your boss can try to intercept the traffic - but when you'll connect to Tor, it will be unable to circumvert the Tor itself - he will need to hack the machine to do so.