Why does my application keeps switching to a different monitor when it regains focus?

Solution 1:

According to this forum post, (at Whirlpool.net, of all places!) FireFox only remembers the current location when it is not maximized:

Firefox only remember[s] which monitor they are on if you close Firefox while it is not maximised.

So procedure to get firefox to open maximised on the monitor of your choice in dual monitor setup:

1. Un-maximise Firefox window
2. Position un-maximised window on monitor of you choice
3. Close Firefox window
4. Reopen Firefox window
5. Maximise Firefox window
6. Close Firefox window

Now Firefox will open maximised on the screen you chose.

Following these steps resolved my long-standing weird issue.

More technical information on this issue can be found at this blog post on Multi-Monitor Programming Pitfalls which reveals it is not solely a FireFox issue.