Hello hmahmud, In order to do that, I will follow the steps below.

1.Go to the Firefox browser homepage.

2.Click on the gear shaped icon on the top right of the screen. It will redirect you to settings(options)

3.Then, uncheck the top site box,

Tell me if that helped and, Stay Safe!

That would remove the top sites list. I do not want that. Suppose there are 10 different pages of YouTube. I have to dismiss 10 times on the YouTube icon. My question is whether there is any way that I could dismiss all of them together simply by dismissing the YouTube icon only once.

You can right click on any open tab to open a tab context menu.

There are several options available. (screenshot).

You can right-click a tab and then select "Close other tabs" or "Select All tabs" -deselect some tabs (Ctrl-right click) and then "Close tabs" to close only those that remain selected.

It is not recommended to dismiss that many page.
Page you dismiss are stored as hashes in a pref on the about:config page that will grow excessively in that case.

• browser.newtabpage.pinned => websites pinned to a tile
• browser.newtabpage.blocked => websites removed by clicking the X (Dismiss) button in a tile

If you have already done this then you can check this pref on the about:config page.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

The "Delete from History" item in the three-dot menu doesn't has this effect and is a better choice in most (all) cases. You can also consider to visit these pages in a Private Browsing window.