How do I fix the space between the bottom of my window and taskbar?

I notice you do not display the Add-on bar. If you switch it on and off, does that make any difference? You can do that by either:

• pressing Ctrl+/
• right-clicking a blank area of the tab bar, or the new tab button ("+") on the bar

Window dimensions are stored in a settings file with your toolbar customizations. This file sometimes become corrupted. It look as though it wouldn't take very long to set your toolbars up again, so you might try this: rename the file and customize from scratch.

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

Help > Troubleshooting Information > "Show Folder" button

Leaving 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 and maximize the window. Does it behave normally?

(If that doesn't help, you could reverse the above process. Exit Firefox, delete the newly created localstore.rdf, and rename localstore.old to localstore.rdf.)

I can't be sure but it looks like you are using the resize-able window, not the full screen window. If that is the case, move your mouse to the bottom of the window and move it slowly until you see an arrow. Using that arrow, hold down your left key and put the window frame where you want it to be.

Nice Work.