I have a Windows 10 laptop and an ultrawide (3440x1440) second monitor, and I scale both monitors to 125% for readability. I use PortableApps but don't "port" it - it's simply installed in a folder on my main drive. I have Firefox Quantum 62.0, Portable of course. I normally keep my Firefox window on the second monitor, and it used to happily open wherever it was when I closed it previously. One day that changed. Now, if it was closed while on that second monitor, it will reopen below and to the right of where it was when it closed (roughly 360 pixels down and 160 pixels to the right). That differential is consistent no matter where on the monitor it was. The window's size is remembered correctly, and I normally use Firefox mostly filling the right half of the ultrawide, so when it does this, it is slid partially off the screen in both axes. Yeah, I can slide it back. Yeah, it's a first-world problem. But it's annoying.
The problem does not occur with any other software, even Thunderbird Portable. (Some software is dumb and always reopens on the primary monitor, but any software that can reopen on the second monitor will do so in the right place.) If I set the scale to 100%, or if it's on the primary monitor (even at 125%), Firefox behaves correctly.
This problem started recently (sometime within the last couple weeks, I think), and I've not changed anything about my monitor arrangement in ages. I don't know if I did something wonky with Firefox right before it started acting this way, but if I did, how do I fix it?