Question: Since I upgraded from Ubuntu 16.04 to 18.04, both Filezilla and Firefox always open in full-screen mode, no matter how I have adjusted them before closing. 

What can I do to change this annoying behaviour?

Answer: 
Either:

1. Double-click on the application's titlebar to minimize/maximize the window.
2. Use the GNOME Tweaks tool, and enable the following, and then click the little square icon in the top-right corner of the application's window.

[![enter image description here](https://i.stack.imgur.com/nihCZ.png)](https://i.stack.imgur.com/nihCZ.png)

**Update #1:**

In `terminal`...

```
cd ~/.mozilla/firefox                # change to firefox prefs directory

ls -al *.default                     # the one with the newest date is yours

cd {.default directory found before} # change to your user directory

ls -alt | grep "Oct 25"              # note which files changed today

```

one of these files is causing your problem...
Problem with Firefox has been solved by software update. Don't know if it was a Firefox problem or something in Ubuntu. Problem with Filezilla always opening at full-screen continues but I can live with that. Thanks for all your suggestions -- it has been a learning experience for me.