I have just installed the new version on top of v. 10.1.4828. My work set-up is as follows: Main screen S1 to the left and Secondary (large) screen S2 to the right.
Firefox’s main window (full height) covers the left half of S2. Under v. 10.1.4828 the main window of eMC was located ± half-height at the mid-right of S2, while single mail windows (compose/reply/forward) as well as the Settings window opened on the right half of S2. These positions were always remembered after a new launch.
After installing v. 10.3, however, these windows open by default on the left side of S2, thus covering the browser’s window and requiring to be moved manually every single time, as I often need to peruse the browser while writing a mail. Not to mention that a bit of bad luck, as of today, can easily lead to smaller catastrophies.
Only exceptions noticed so far: the Accounts window, which still opens as set up earlier, on S1; the Operations window, which at any rate was already located at the bottom extreme left of S2; and the Reminders window, which used to be at the bottom extreme right of S2 and has now moved to the bottom right of the left half (if absolutely necessary I’ll try to post some screen shots.)
Actually it looks a bit as if the application now understood the 2560x1440 S2 screen to be a 1280x1440.