Gfilui

This does not copy the file.
I am owner of both directories.

And there are no messages?

cd /home/andy/.mozilla/seamonkey/4e39n1np.default/
cp -f bookmarks.html /home/andy/.mozilla/firefox/mltw8z53.default-1516629000620/

File exists. File can't be moved in file explorer. I even deleted the "Firefox bookmarks file" and copied Seamonkey bookmarks file there??

Adding the output of namei -

f: /home/andy/.mozilla/seamonkey/4e39n1np.default/bookmarks.html

Drwxr-xr-x root root / 
drwxr-xr-x root root home 
drwxrwxr-x andy andy andy 
drwxrw-rw- andy andy .mozilla 
drwxrw-rw- andy andy seamonkey 
drwxrwxrwx andy andy 4e39n1np.default 
-rw-r--r-- andy andy bookmarks.html 

f: /home/andy/.mozilla/firefox/mltw8z53.default- 1516629000620/bookmarks.html

Drwxr-xr-x root root / 
drwxr-xr-x root root home 
drwxrwxr-x andy andy andy 
drwxrw-rw- andy andy .mozilla 
drwxrwxrwx andy andy firefox 
drwx------ andy andy mltw8z53.default-1516629000620 
-rw-r--r-- andy andy bookmarks.html

@wave5459

andyk_~/Downloads$ rm /home/andy/.mozilla/firefox/mltw8z53.default-1516629000620/bookmarks.html
andyk_~/Downloads$

File is deleted. But Firefox recreates it. :-(

Maybe the simplest resolution you seek isn't so much Ubuntu related as it is Firefox related . You want to import your bookmarks from Seaborn to Firefox I presume.
Here's a suggested response on a link I found that seems to have worked in that case -

If you have a SeaMonkey version that uses places.sqlite then you can
copy this file from the SeaMonkey profile folder to the Firefox
profile folder.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_SeaMonkey

Source - https://support.mozilla.org/en-US/questions/963390

Let me know if it works.