You can find a RSS (Subscribe to this feed) icon in the Customize palette that you can drag on a toolbar if that is what you mean.

• https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars
• https://support.mozilla.org/kb/navigation-buttons-missing

No. Simply some Firefox site' s URLs are unvisible at (trought) start.me agregator.

That is a problem with that specific content on that page.

• https://start.me/p/V0Av46/iv-high-tech-news

The page opens some links in an iframe and all Mozilla servers send an "x-frame-options: DENY" item in the HTTP response headers that instructs the browser not to load the page(s) in an iframe.

• https://developer.mozilla.org/Web/HTTP/X-Frame-Options
• https://addons.mozilla.org/ru/android/extensions/?sort=created&page=1

You will have to open each of those pages in their own tab or use an extension that supports splitting a tab to show multiple pages.

But URL https://marketplace.firefox.com/new has visible content through agregator start.me alike RSS.

Do not let the presence of Firefox in the TLD confuse you. The marketplace.firefox.com website is not an (official) Mozilla website and also doesn't send the above mentioned x-frame-options header. Mozilla websites are usually hosted on the mozilla.org domain.

Fishing forever?

For answer "Do not let the presence of Firefox in the TLD... " thank you very mach.