First my setup: Mac pro desktop, 10.5.8 2Gb memory, spaces enabled (multiple desktops) 2 monitors, main monitor left hand side Firefox 3.6 upgraded to Firefox 12 as an attempt to fix problem I almost always have Firefox open with 2 windows, one on each monitor Also Safari in another space also with 2 windows, one on each monitor Often Adobe Lightroom in another space. I know I need more memory. I do a lot of photo processing and publishing to Flickr. Often after working for a few hours I have to shut things down and start again as it grinds to a halt. Normally with Firefox windows open on the same monitor that I click on and if I open a few small windows they open in random places on the monitor. Then tonight they all started opening in the top left corner of the main monitor, even when I click a link on the other monitor. No amount of moving a window, resizing, maximising, deleting, reopening made any difference. This has happened twice before and just randomly playing with the window (resizing, moving, deleting, etc) seemed to solve the problem. But tonight it did nothing. This only happens with Firefox. Safari is still behaving normally. I upgraded to Firefox 12 and it made no difference. I found some comments on corrupted localstore.rdf. Followed their instructions to 'Reset toolbars and controls', then tried 'Reset all user preferences to Firefox defaults'. Neither made any difference. Then tried to copy my backup of localstore.rdf thinking it would put everything back as it was, but that failed as well, so I had to manually adjust everything again. I have no idea now how to get windows to open on the same monitor again. Any suggestions would be appreciated.