By full screen, do you mean what happens when you press F11 or are you using a button in the page, for example, on a video player?

What do you mean by "re-opening it from another program"? Do you mean you exited out of Firefox, and instead of starting it from its own shortcut, you are starting it from an external link, for example, in a mail program or in Acrobat?

"Full screen" refers to the mode you can choose with the three symbols in the upper right corner of a window. The right of these three symbols closes the program, the left one minimizes and the middle one toggles between "covering the entire screen" and "smaller with adjustable size". No F11 is involved.

I think, what I called "full screen" is actually called "maximized" and what I first named "maximized" is called "restore down" (my apologies for that). I just confused both as I am operating Firefox in the "smaller with adjustable size"-mode and made the window almost cover the entire screen (which is the best I could do without getting automatic resizings)

Re-opening from another program means, I have another program in the "covering the entire screen"-mode (e.g. Thunderbird when writing en e-mail) and switch to Firefox by hovering over the symbol in the task bar (such that the different tabs appear above it) and then selecting the corresponding tab. The selection of the tab is exactly when Firefox automatically "restores down" a "maximized" window.

Okay, I think I understand now:

• You are using a maximized window in Firefox
• You switch to another application (for example, by clicking its Taskbar button or using Alt+Tab) that also is using a maximized window
• You switch back to Firefox by hovering over its Taskbar button and clicking either a thumbnail or a list item for an open tab [I assume you are using the setting "Show tab previews in the Windows taskbar" so that you can see more than one tab per window in the list]
• BUG: The Firefox window visibly changes from a maximized window to a resizable window during the switch

I think that definitely should not happen, and I don't have a good theory as to why it is happening.

Exactly, you understood it perfectly right.

There must be some code lines which tell Firefox to read the resizable window dimension and position from the xulstore file, and to resize the window correspondingly. The problem with windows not being maximized as expected seems to be quite common (although the situation is in most cases a fresh start of Firefox rather than a change from another program):

• https://www.reddit.com/r/firefox/comments/6ltq18/firefox_window_sizing_problem_w_xulstorejson/
• https://support.mozilla.org/en-US/questions/1149222
• https://support.mozilla.org/bm/questions/1228707
• https://www.reddit.com/r/firefox/comments/dy8ovu/new_windows_arent_maximized/

EThe problem with windows not being maximized as expected seems to be quite common (although the situation is in most cases a fresh start of Firefox rather than a change from another program).

Those threads are different:

(1) New windows use a "default" size when you set privacy.resistFingerprinting to true in about:config -- I don't see anyone reporting that this causes maximized windows to resize when you switch to them (also, you are not using this setting)

(2) Maximized state not restore at startup because sizemode was not updated to maximized in Firefox 50 on Linux -- does not appear to be a problem on Windows 10 today

There was an interesting hint here:

I've found that new windows are only started maximized if no other Firefox window currently has fullscreen video (i.e. Netflix). Source: https://www.reddit.com/r/firefox/comments/dy8ovu/new_windows_arent_maximized/f808oid/

However, I don't know whether you have any full-screened media windows open and the user didn't mention that an existing window would change from maximized to resizable.

So your bug sounds like something new.

If you keep your old xulstore.json file (you can rename it xulstoreOLD.json) when creating a new one, when you compare the files between one associated with the problem and one that works normally, do you notice any specific differences?

I use the program WinMerge for this kind of comparison, although it can be a bit hard to follow when a file is just one long line... https://winmerge.org/

To start with, the bug appears without any media in fullscreen mode.

However, it only appears, when changing from any other (non-Firefox) program in "maximized" mode to Firefox. If the window of the other program is not "maximized", then Firefox stays in the "maximized" mode when changing to it.

Concerning the xulstore, I do not have an example of a working one. The only thing I noticed is that after making Firefox maximized, the 5th entry in the xulstore file "sizemode": is always "maximized" (I guess this is how it should be). After triggering the bug (which resizes the window) and while the window is still resized, the entry reads "sizemode":"normal". This is the only difference occurring under the bug. Of course, one can also change the "screenX" and "screenY" variables by shifting the resized window around. But the "sizemode" seems to be automatically set to "normal" by the bug.

The problem also only seems to appear when changing to Firefox by hovering over the task bar and then selecting one of the appearing thumbnails of the tabs. It does not occur under program change by Win+Tab.

Have you noticed any popup or pop-under windows opening? Wondering whether that might change the sizemode. ??

I suggest filing a bug: https://bugzilla.mozilla.org/

Nope, there have been no pop-ups or pop-unders. The window just resizes.

I submitted the bug to bugzilla now (https://bugzilla.mozilla.org/show_bug.cgi?id=1717456)

Until there is a solution, I guess it might be a temporary solution to keep the window in "normal" mode and resize it such that it almost covers the entire screen.