To do this, in Firefox click on Click on Menu -> Help ->  More Troubleshooting Information or navigate to about:support in the address bar.

Under the Application Basics section, there will be a section called Profile Folder with a button to Open Directory.

In the Profile Directory create a new folder called chrome. In the chrome folder create or edit the file userChrome.css if it already exists.

The contents of userChrome.css should be the following.

/* hides the native tabs */
#TabsToolbar {
visibility: collapse;
}