Is it plagiarism if my logo looks similar to an existing one?

I'm designing a logo inspired by the MDN logo.

My logo (inside the green zone) has nothing to do with the "dinosaur" logo of Mozilla. So the question is about the frame and the arrangement of the letters (IJK in my case).

Is it plagiarism if my logo looks similar to an existing one?

Answer

I was previously familiar with the MDN logo, but no your logo did not come to mind when I saw yours. Is it plagiarism? Yes, if you used the MDN logo for inspiration then yes I'd say that's plagiarism. But not a very serious case of it.

Is it highly unethical? It depends, but probably not. A rounded rectangle with a thick black stroke isn't exactly groundbreaking design work. Is it at all unethical? Maybe a little. Mimicry is widespread in the design world. Without it, we wouldn't have design trends. If it helps you sleep at night, you can refer to it as "inspiration" instead of plagiarism.

Is it a copyright violation? I'm not a lawyer, but I'd say no. Here's a good article on this sort of thing:

In order for a work to have copyright protection, it must reach a requisite level of creativity. Many logos, however, do not. Since copyright can’t protect a name, colors or the design of the logo, most simple logos simply do not have the required level of creativity to be considered copyrightable. However, many ornate or artistic ones do.

What we have here is not particularly ornate or artistic. As long as your omitted logo is 100% yours, I think you're on the safe side of things.

There are lots of existing logos out there that share similarities, for companies that are presumably much bigger than yours.