I want to let the firefox window always maxmized at startup,not full screen. But I don't want to use Javascript or something else to implement it. I want to know if there is a simple option setting in the about:config to achive this? Thanks in advance,


There is no option in about:config to start firefox in fullscreen.

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/change the sizemode="fullscreen" parameter.

<RDF:Description RDF:about="chrome://browser/content/browser.xul#main-window"
        sizemode="fullscreen" />

To access Firefox profile folder, type about:support on addressbar and find option to view the folder under Application Basics section. The option may be "Show Folder" (Windows), "Open Directory" (Linux) or "Open Containing Folder" depending on OS and version of firefox.

Close firefox before editing the file. Otherwise the change will be reverted.

NB: The original author of this answer disclaims that he/she is not sure about its working on Mac.

  • thanks for your reply. But I don't want the full screen mode, and I just want the Firefox window maxmized.
    – Ryan
    Feb 17 '14 at 9:33

Another way is to edit the xulstore.json file as described here: https://gist.github.com/hoangtranwork/fbd308f326a741109f8eb03a9990a839

  1. Get the profile folder location in about:support page

  2. Edit the file xulstore.json found in the above folder. It's a json file and we need only to edit the key main-window to something like this: "main-window":{"screenX":"0","screenY":"0","width":"1920","height":"1080","sizemode":"maximized"}

NOTE: - Step 2 should be done when Firefox is not open, as it seems that the file get updated when Firefox closes. - Sometimes you need to put negative value to screenX if the windows is still shifted abit from the screen edge.

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