How can I disable the DevTools that pops up from pressing F12?
There is very good reason for wanting to disable the devtools: On many keyboards it is very easy to press F12 when trying to press F12. That causes devtools to come up and hog half of the screen space and take the cursor of the text field you were editing. That can be quite annoying.
To add to the insult, the DevTools is something that is only needed if you develop websites. Regular users don't need it. Quite a lot of people are annoyed by it (Just google up "firefox disable F12" and see. It is counterintuitive to install a plugin just to remove a function that very few people need and that many people are annoyed by.
Or is Firefox mainly made for web developers and other people should just use something else?
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.