So... the add-on solves your problem, or you are looking for another approach?

Firefox allows you to apply custom style rules to modify the interface. In a little test, this rule worked for me:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
#navigator-toolbox[inFullscreen] #PersonalToolbar{
  visibility: visible !important;
}

You can apply custom style rules to the interface using either:

• Stylish extension (https://addons.mozilla.org/firefox/ad.../stylish/)
• userChrome.css file (http://kb.mozillazine.org/userChrome.css)

Another possible code is this code to the toolbar visible that are enabled in normal mode.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

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

/* full screen toolbars */
#navigator-toolbox[inFullscreen] toolbar:not([collapsed="true"]) {
 visibility:visible!important;
}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userChrome.css file in the editor window
• Make sure that the userChrome.css file starts with the default @namespace line
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css.
  Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox