The New Tab custom wallpaper (and colors) option is available since version 139. Your own image can be uploaded as your New Tab wallpaper or any custom color can be selected - from the brightest pink to dark gray. This feature will be rolling out gradually to new users and can also be enabled immediately via Firefox Labs.

그걸 어디서 어떻게 찾아 작동시키는지 알려달라고

Do you have all of these buttons, including "Upload an image", or are you missing some of them?

If your image doesn't work well with the design of the page, you should be able to fix that using either:

(A) Click the upload button and choose a different image file.

(B) Click one of the other background options.

If you have the first four buttons but "Upload an image" is missing, you might have disabled "Studies" in your Firefox. Mozilla uses Studies to roll out new features progressively, before turning them on for everyone in a future version of Firefox. This article shows where to find your Studies setting: About Studies

Either way, here's how you can turn it on manually right now:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste customWallpaper and pause while the list is filtered. Firefox 139 should list a few different preferences.

(3) Double-click the browser.newtabpage.activity-stream.newtabWallpapers.customWallpaper.enabled preference to switch the value from false to true

Please don't manually edit the other preferences.

그 화면으로 들어가는 톱니바퀴 모양 아이콘 자체가 삭제됨

Is it a default Firefox new tab page? Could you attach a screenshot?