I Googled by search word, "How to delete bookmarks saved in iCloud" and which hit the following:
1. Turn on Bookmarks and update your device. Tap Settings > [your name] > iCloud and make sure that Safari is turned on. ...
2. Check your Internet connection. Open Safari and go to
http://www.apple.com. ...
3. Check your account details. ...
4. Restart Safari. ...
5. Turn off iCloud Bookmarks and turn it back on. ...
6. Restart your device.
In 1. above, Safari in my device is presently turned off so that I have to first turn on, then message said the present bookmarks on the device(iPhone) would merge with that in iCloud. I went ahead to let that done.
I do not see any change to be done in 2. and 3.
In 4. I assume Safari means an internet browser which in my case is Firefox.
In 5. I assume it means to launch iCloud app on my W10 and remove check mark on he Bookmarks and put it back on as shown below.
My problem is that after putting the check mark and click on "Apply", then a message prompt says download and install iCloud Bookmarks extension on Firefox even though the iCloud Bookmarks Extension is installed on the Firefox. I tried installing it many times but the same message returned. Another word I cannot turn on iCloud Bookmarks..