It's strange that it starts out maximized before flipping to the smaller size. ??

If instead of maximizing the small window you stretch it out by dragging the right border or lower right corner, does Firefox remember that new size or does it go back to the smaller size?

The window does not stretch. The cursor doesn't change also as the window "thinks" it's already maximized. All of the four corners and the four sides are un-stretchable and nothing happens if I try to drag them by force. It is however possible to drag the window by the tab area (title bar), at which point the window "realizes" that it's not maximized. After that the window can be stretched out and also properly maximized.

Hmm, I can't think of any good reason for that, unless Firefox is for some reason thinking that an old popup window is an appropriate model for window behavior.

Sometimes the settings file that stores window dimensions becomes corrupted with nonsensical values. You can rename the file and have Firefox rebuild it. Here's how:

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

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows File Explorer.

{MAC} In the first table on the page, on the Profile Folder row, click the "Show in Finder" button. If Finder highlights an icon with a semi-random name like a1b2c3d4.default, double-click it to display the contents of that profile folder.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then:

Right-click xulstore.json then Rename to something like xulstoreOLD.json

Start Firefox back up again. Do windows display normally again?

That mini-reset may hide the bookmarks toolbar and/or menu bar. You can use one of these methods to show the toolbar list and select the desired bars there:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key or F10 key to activate the menu bar temporarily.
• in Customize mode > Toolbars button (see: Customize Firefox controls, buttons and toolbars)

To turn on the Bookmarks Toolbar or Menu Bar, click it on the list.