Install CompizConfig Settings Manager.

sudo apt-get install compizconfig-settings-manager

Launch it by searching from the dash in Unity, or Preferences > CompizConfig Settings Manager in Ubuntu Classic

• Navigate to Place Windows
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• Then Navigate to the Fixed Window Placement Tab
• Add a rule to fixed placement mode
• open FireFox and size it to your need
• Use the Grab button to “grab” the window name and size
• 
• Your rule should look like this when done
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