Doesn't Firefox keep the size if you change the main browser window and close the Firefox application?

Do you have a multiple monitor setup?

See:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Try to remove the Firefox plist file (org.mozilla.firefox.plist).

• http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results

Go to "~/Library/Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist)

I simply don't know how to resize it in the first place. In searching online for a solution I saw people were having problems keeping it resized so I added that to my question.

Make sure that the Firefox window isn't maximized and that you do not run Firefox in full screen mode (command+Shift+F).

Then you should be able to resize the Firefox window with the left mouse button held down (e.g. in the corner at the bottom right).

I didn't know about the small resizing triangle in the bottom right hand corner of the Firefox browser! Thanks for solving my problem!