The initial window size is enforced if you have privacy.resistFingerprinting set to true in about:config.

Does it still happen with all your extensions disabled?

zeroknight, the issue occurs at random intervals but roughly 2, sometimes 3, times a week and I wouldn't want to disable all of my extensions for days.

Setting privacy.resistFingerprinting to true enforcing the initial window size is not the desired outcome. I want to keep my current, manually adjusted window size (that I have been using for months) as the default because I need it to be a specific size.

This can happen if there is a problem with saving window data to xulstore.json in the Firefox profile folder with the result that the window size gets reset.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

cor-el, I usually use the Ctrl + W shortcut (macro key) to close out of Firefox. Is that recommended against? There's usually just one open + active tab when I do that. Never multiple windows.