Convert the popup window back to a tab, and insert back to original window.
The tab is actually "moved" between the normal window and the popup window, so you don't have to reload the page.
The popup window size is maintained based on the domain. You can also right-click in the popup window and make it remember the window size for the entire URL.
Usage
Right-click the tab bar, select "Popup this tab". (Firefox 53+)
Right-click on the page, select "Popup this tab". (Firefox < 53)
In popup window, right-click on the page, select "Merge popup".
From Firefox >= 66, you can set a hotkey that will popup/merge the current tab. Learn more
