To add on to this, if what you are saying is constructive, meant to be meaningful/helpful, and is true- then what fears of getting fired should you have?

Even if you're just bitching about some thing/person/policy that gets on your nerves- if what you're saying isn't slanderous and is true, why should you fear repercussion?

If being candid with your boss means you have to worry about your job, you would probably be happier elsewhere.