There's a button called "Solved It" next to each response to a question. Click that.

I used the solved button on this question, thanks. On my question which I want to say solved, https://support.mozilla.com/en-US/questions/825804, I see only edit options, not solved button. Also, I viewed several posts with replies and observed only the helpful buttons. Where can I get more information on this process?

You can't respond in that way if you solved your own problem.

The easiest thing to do in this particular case is to post a note in your thread asking a moderator to lock it for you.

Otherwise, click the link to "Dashboard" top right of this forum and it'll take you to the forums which deal with how the site works.