Reset firefox to fix problem

At the top of the Firefox window, click the Firefoxbutton, go over to the Help sub-menuand select Troubleshooting Information.Click the Reset Firefox… button in the upper-right corner of the Troubleshooting Information page.To continue, click Reset Firefox in the confirmation window that opens.Firefox will close and be reset. When it's done, a window will list the information that was imported. Click Finish and Firefox will open.

If the page uses a script to specify certain window dimensions, then Firefox usually will honor those dimensions.

However, if the new window is launched without that information, the window typically will have the same size/state as the current window, or maybe the last closed window.

The information about window sizes and positions is stored in the same file as toolbar customizations. You can get rid of this file without doing a full Firefox reset. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leave that window open, switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Restart Firefox. Hopefully now your window behavior will be back to the default.

What is the default window size when you start Firefox?

If Firefox is maximized then un-maximize the window and set the size to you liking.
Close Firefox to save that as the default setting and check if this has worked by restarting Firefox.