When Morrowind was originally released (around 2002), monitors had 4:3 (physical) aspect ratio and Morrowind used 75 degree aspect ratio.
Now, most people use widescreen displays at 1680x1050 or 1920x1200 with a 16:10 aspect ratio. 16:9 monitors with resolutions like 1920x1080 are a bit less commonly used (more info here: http://en.wikipedia.org/wiki/Display_resolution)
I'm going to assume only square pixels, which simplifies the calculations. Using oddball resolutions like 1280x1024 on a 16:10 montor is going to confuse things as it involves rectangular pixels.
Under these assumptions, a 16:10 widescreen user (typically native 1680x1050 or 1920x1200) should use
2 * arctan( 16/10 * 3/4 * tan( 0.5 * 75 ) ) = 85.277... degrees
and a 16:9 widescreen user (typically native 1920x1080) should use
2 * arctan( 16/9 * 3/4 * tan( 0.5 * 75 ) ) = 91.308... degrees
Note: in these calculations the functions arctan and tan involve degrees, not radians.
The use of trigonometric functions is clearly necessary because otherwise a wide enough screen would lead to a calculated result above 180, which leads to the nonsensical conclusion that you should be seeing things behind you on your flat monitor. tan(FOV) is also nonsensical as a 90 degree FOV is perfectly valid but tan(90) cannot be computed. Instead it is tan(FOV/2) that should be used. FYI the tangent of 75/2 is simply the ratio of game distance between the center of the 4x3 screen and the side, divided by the game screen-to-camera distance. This distance is simply scaled and converted back to FOV angle for use on a widescreen.