Here, I found one answer from the internet for you:

Open Firefox Access about:support (or click on Menu > Help > Troubleshooting Information) See the “Profile Folder” there, under “Application Basics”? You can open that path on Explorer/Finder/GFiles, or just click on ‘Open Folder’. Read more about profile folders here if you’re so inclined.

Create your userChrome.css Now that you’re in the profile folder, create a new folder named “chrome” . Just chrome, lowercase.

Inside that folder, create a new file named “userChrome.css“, with the following content:

#content browser{
  margin-right:-17px!important;
  margin-bottom:-17px!important;
  overflow-y:scroll;
  overflow-x:hidden;
}

Let’s check: this file should be at [wherever your profile folder is]/chrome/userChrome.css .

Save it, and restart Firefox. You’re done! The scrollbars are gone.

I hope this answer helpful to you.I tried in my computer and it worked

Try this code in userChrome.css (you may have to adjust the margin values):

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

:-moz-any(#content,#appcontent) browser{
 margin-right:-14px!important;
 overflow-y:scroll;
 margin-bottom:-14px!important;
 overflow-x:scroll;
}

@Trupesh thank you for your resourceful information it give me much more insight about profile folder. but unfortunately your code snippet doesn't work.

@cor-el thank you the code works like a charm.

thank you everyone for your fast answer