I am using Thunderbird 140.8.0esr (64-bit) on Kubuntu 25.10 multi-monitor configuration. New install, very little customization. Thunderbird will not retain the window size or position for the main window or messages. I have have tried deleting both the xulstore.json and session.json files (one at a time and both). When I delete the xulstore.json file and restart Thurderbird, it starts maximized. When I set the position manually e.g. to use about 90% of the screen for the main window and about half the width and about 90% of height for a message window and then close Thurderbird I can see the new window size vales written into xulstore.json e.g 2493 x 1362 and 1501 x 1317 "messengerWindow":{"sizemode":"normal","screenX":"0","screenY":"0","width":"2493","height":"1362"} "messengerWindow":{"sizemode":"normal","screenX":"0","screenY":"0","width":"1501","height":"1317"} When I close and restart thunderbird it reverts to opening in the center of the primary monitor at a smaller size than when I closed it. The values in xulstore.json have changed to 960 x 540 "messengerWindow":{"sizemode":"normal","screenX":"0","screenY":"0","width":"960","height":"540"} "messengerWindow":{"sizemode":"normal","screenX":"0","screenY":"0","width":"960","height":"540"} I found a similar recent question if this helps with a possible solution: https://support.mozilla.org/en-US/questions/1527783 but I cannot locate the referenced userChrome.css file to see if there might be something similar to try. I am also using a multi-monitor setup. If I move the Thunderbird window to the second screen and close it, it always re-opens on the primary display at the 940x540 size. Thanks!