F keys have been tied to special functions since the beginning of Software, and F12 is no exception. I don't see anything wrong with this behavior, you accidentally hit the wrong key, something happens. It's no more wrong than when you are typing and you accidentally hit "a" instead of "s".

Now, if you want to fully remove devtools, that's another problem altogether. It's not currently supported in current versions of Firefox, but may be in the future. In the meantime I'm pretty sure you can install an add-on to disable devtools.

That's pretty much what I expected you to answer.

Yes... I could install a plugin for it. Or then I can just install another browser.

I know several people who are annoyed by that F12 -function in FF (my wife for example). I know that basically the annoyance is caused by accidental keypress, but it seems to be one that happens quite often and is a lot more annoying than accidentally pressin "a" instead of "s"... When you press a wrong letter, you can just press backspace, and continue writing... When you press F12 in FF you have to use the mouse to continue what you were doing.

It wouldn't probably be too hard to make an about:config entry for disabling the DevTools (or just the F12 mapping). Most people wouldn't even notice if it was disabled as default exept some (mostly laptop users) would be annoyed less often. I understand that it works as designed... The way it is designed just seems to annoy some people like me.

So the question still is if Firefox is mainly made for web developers and other people should just use something else?

Firefox is for all users, and like I said, there will likely be a version of Firefox in the future that comes without devtools. For the time being, that is not the case and until there there are add-ons that will disable the shortcut or devtools entirely.