See this KN article - https://support.mozilla.org/en-US/kb/search-contents-current-page-text-or-links

That does not address what I meant. That is searching a current page you are viewing. I meant searching your list of bookmarks (say 200 bookmarks) for content.

A bookmark is a shortcut to a specific webpage, along with the favicon data (the website's image that is saved within the "bookmark").

The "content" of a bookmarked webpage is not contained within the actual bookmark. To search the "content" you need to view the page thru browsing history to see the webpage the last time you viewed that bookmarked page, or by viewing the current page by accessing it via the bookmark.

Once again, the "bookmark" is a shortcut to the actual webpage; "page content" isn't saved in the bookmark.