First, I wouldn't say anything negative about the company or my experience there. As you observe about your previous manager, people don't like negative people. Your prospective employer won't appreciate hearing you complain. Negative people are also disproportionately likely to be the source of problems themselves, so complaining a lot about your previous experience will be waving a red flag that you are a potential source of problems.
When discussing your previous positions stick to facts (I did X, Y, and Z) and positive notes about what you learned and accomplished.
Second, I wouldn't tell people that I was fired. I would list a year of experience there. If someone asked why I left I'd say that I learned a ton, the work was interesting - whatever true and positive evaluations I had, but ultimately I wasn't sure I could accomplish my career goals at the startup and so, after giving it a year, I've decided to consider other opportunities.
When discussing your previous positions stick to facts (I did X, Y, and Z) and positive notes about what you learned and accomplished.
Second, I wouldn't tell people that I was fired. I would list a year of experience there. If someone asked why I left I'd say that I learned a ton, the work was interesting - whatever true and positive evaluations I had, but ultimately I wasn't sure I could accomplish my career goals at the startup and so, after giving it a year, I've decided to consider other opportunities.