On Mac, Command+Shift+f should also switch between full screen and normal view.

Another method that might work is to slide the mouse pointer all the way to the top of the screen. If the tab bar slides down, you can right-click (Ctrl+click?) a blank area of it (or right-click the "+" button) and you should get a short context menu with the option to Exit Full Screen Mode.

Does that work?

Yes, that worked! All I had to do was click on command+shift+f to clear it up. I've been struggling with this for a couple of hours, what a relief. Thanks very much!

Glad to hear that worked.

If Firefox keeps starting up that way, there might be a problem with one of Firefox's settings files. Since the file is hard to fix, we usually recommend that you rename it and have Firefox start a new one. I'll mention the steps here for future reference just in case.

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

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

In the first table on the page, click the "Show in Finder" button. This should launch a new window listing various files and folders.

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

• "3-bar" menu button > "power" button
• (menu bar) File > Exit (or on Mac it might be Firefox > Quit)

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old.

Start Firefox back up again and it should use a resizable window and hopefully not get stuck in full screen view again.

Great, I've made a copy of this to have on file. I've restarted firefox a couple of times and it's been opening normally, so hopefully it was just something I clicked on accidentally that set it to full screen. Thanks very much again!