Hello,
A while back I evaluated a Palo Alto firewall by placing it in TAP mode on our network. One of the features that I liked was that it could log web history for each employee on our network. However it has come to my attention that many of these firewall appliances are really just Linux servers with custom software. I have been searching for open-source alternatives that can accomplish the same thing for free.
I have already tried NTOP but it was not exactly what I was searching for. I also tried Security Onion but have not been able to get it to work properly. Someone in another forum pointed me to Squid proxy server but from the way it was being described, it wouldn't be the best fit for our network.
Any suggestions?

Install a squid-proxy with single-sign-on for the domain-accounts and log everything as you need.

Depending on where you are you should parallely state a job-instruction that the devices of the company may not be used for private content. In a lot of countries: When you allow private web-browsing this can be a legal nightmare because you are technically then an ISP for your employees and have to protect their personal rights.