Hi, if you are referring to the New Tab page, you can pin sites that you don't want to be replaced, by pointing to the top right-hand corner of the icon, then clicking the 3-dot menu that appears (...). Please see Customize your Firefox New Tab page.

If that doesn't solve it, one of your extensions may be causing this: Hold down the Option key when you start Firefox to start in Safe Mode. More - Use Troubleshoot Mode in Firefox.

If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions, then restart Firefox; now if it's still OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

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Will clarify. This answer is for top sites, I am talking about bookmarks. I can't turn off some extensions, because bookmarks usually dissapear after week or so.

You normally see history items on the Firefox Home pages and even bookmarks that appear need to have a visit count that is high enough to include them. Like posted above, you can pin specific items like a bookmark or a history item or a custom URL to a tile to ensure that it isn't replaced by another history item with a higher visit count (actually frecency bonus points are used).