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 - Diagnose Firefox issues using Troubleshoot Mode.

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.

If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!

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).