No you can't modify the firmware in the beta version. It allows you to help test the latest firmware and help debug it so they can push a stable release out to everyone. The more people who test the faster any bugs get found. If you want to help request an ota for the Beta, I've not found any issues so far so think it's pretty safe to install as a daily driver. If you find a bug report it via the feedback app.
Also many people use the Beta just to have the latest version. Unless users actively hunt down bugs and report them through the feedback notification then potentially the stable version could be buggy. I'd suspect bugs are prioritised in the ratio of beta downloads vs number of reports.