clear cache and cookies and make sure you're using the latest version of Firefox

Is that about the Firefox Home page with the tiles? *https://support.mozilla.org/en-US/kb/customize-new-tab-page You can style the Firefox Home page via CSS code in userContent.css. *[/questions/1410281] How to customize about:newtab (using a wallpaper)?

@-moz-document url-prefix(about:home), url-prefix(about:newtab) {
  body {
   background: url(background-image.jpg) !important ;
   background-size: cover !important ;
  }
}

Add CSS code to the <b>userContent.css</b> file in the chrome folder in the Firefox profile folder. You can place the background-image in the same chrome folder as userContent.css or open the file in a Firefox tab to get the file:// URI. <hr> <pre><nowiki> @-moz-document url-prefix(about:home), url-prefix(about:newtab) { body { background: url(background-image.jpg) !important ; background-size: cover !important ; } }</nowiki></pre> <hr> It is not that difficult to create <b>userContent.css</b> if you have never used it. The first step is to open the "Help -> Troubleshooting Information" page and find the button to access the profile folder with the random name (xxxxxxxx.default-release). *https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox You can find the button to go to the profile folder under the "Application Basics" section as "Profile Folder -> Open Folder". If you click this button then you open the profile folder in the Windows File Explorer. You need to create a folder with the name <u>chrome</u> in the profile folder with the random name (name is all lowercase). In the <b>chrome</b> folder you need to create a plain text file with the name <b>userContent.css</b> (name is case sensitive). In this userContent.css text file you paste the text posted. On Mac you can use the TextEdit utility to create the userContent.css file as a plain text file. In Windows saving the file is usually the only time things get more complicated because Windows can silently add a .txt file extension and you end up with a file named <i>userContent.css.txt</i>. To avoid this you need to make sure to select "All files" in the dialog to save the file in the text editor using "Save File as". You need to close (Quit/Exit) and restart Firefox when you create or modify the userContent.css file. More info about userChrome.css/userContent.css in case you are not familiar: *https://www.userchrome.org/what-is-userchrome-css.html *https://www.userchrome.org/how-create-userchrome-css.html *https://www.userchrome.org/firefox-changes-userchrome-css.html In Firefox 69 and later you need to set this pref to true on the <b>about:config</b> page to enable userChrome.css and userContent.css in the chrome folder. *about:config => toolkit.legacyUserProfileCustomizations.stylesheets = true *https://support.mozilla.org/en-US/kb/about-config-editor-firefox