https://support.mozilla.org/en-US/kb/use-bookmarks-firefox-android

Favorites is what Bookmarks are called on Internet Explorer.

My sites are added to the reading list, but not in the bookmarks.

You've accessed Reader Mode on that particular article, so you'll only be able to add to your reading list (treat that like a bookmark). You can add the site to your bookmark list by backing out of Reader Mode, and tapping the toolbar by accessing the menu on the top right (the three dots), and selecting the star icon to bookmark the site.

Hi! I found the menu to add bookmarks, the bookmark is added, but no one knows where is not defined is added to a bookmark. I have many bookmark folders on desctop done and I do not know where the bookmark is added.

In your screenshot, the top item in the menu is denoting that it is not bookmarked. You'll need to tap that item so that it becomes check-marked. You can access your bookmarks from tapping the URL-bar and selecting the Bookmarks tab. In your case, 'Закладки'.

Hi! If I click on the "Закладка" (Bookmark), the message goes, "Закладка добавлена" (Bookmark added). But what if I want to add a bookmark to a folder in bookmarks of desctop? My tablet's bookmarks and my desctop bookmarks are synchronized.