Firefox should open in full screen mode if you close Firefox (quit the Firefox application) while in full screen mode. That works on Windows and Linux, so I would assume that this will also work on a Mac.

Firefox didn't open in full screen mode when I quitted in full screen mode and open it again, it went to normal mode when I opened it again

I'm not on a Mac, so I don't know if you can achieve this.

You can try to edit the localstore.rdf file in the Firefox profile folder and find the section with "browser.xul#main-window" and add the sizemode="fullscreen" parameter.

<RDF:Description RDF:about="chrome://browser/content/browser.xul#main-window"
        height="843"
        width="1145"
        screenX="527"
        screenY="19"
        sizemode="fullscreen" />

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

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

It doesn't let me type or add changes to the "chrome://browser/content/browser.xul#main-window"/> The words are also blue but I can't click on it or change anything

Did you open this file in a (plain) text editor while Firefox wasn't running (Firefox > Quit)?

Did you check that the file wasn't read-only and that you have write permissions?

I opened it in TextEdit and changed it to the number you provided, I saved it and when I quit FF and re-open it, it's still on 'maximized' mode and not fullscreen. I opened the .rdf and the number didn't change but it did change when I opened it in TextEdit

You do not need to change numbers. Those do apply when the Firefox isn't maximized (they show my Firefox window dimensions and position). What needs to be done is to add the sizemode="fullscreen" attribute to this section to make Firefox switch to full screen mode.

Still no luck, added the fullscreen to .rdf and not open in fullscreen.