Firefox doesn't have a toolbar that include such buttons.
The editor on the web page need to supply such buttons just like you see on this forum just above the text area where you enter a reply.

Where can I find these icons for all kinds of writing beside just this reply? Haven't even seen this "reply" form before.

They don't exist, because not all text fields on all websites are even aware of what 'bold' or 'italic' text means. The browser can't force the issue, the website has to provide that facility.

Some extension:

• Text Formating Toolbar: https://addons.mozilla.org/firefox/addon/text-formatting-toolbar/
• BBCodeXtra: https://addons.mozilla.org/firefox/addon/bbcodextra/

I downloaded the toolbar, but it's not always available to use. It's available as I write this message, but disappears when I go to Google, or other websites.