Is it possible that you are clicking inadvertently on the blue star in the address bar? I can't think of any other way unwanted bookmarks would be created.

It is possible to remove the blue star with a file userChrome.css which you have to create. More information if you want it.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Make sure you aren't pressing Ctrl+D by accident ("Bookmark this Page" shortcut).

Try disabling your extensions in case one of them is automatically creating bookmarks.