When Opera IDs itself as IE or Firefox, it still contains the word 'Opera' somewhere in the UA string. That way, if webmasters are checking the stats, they should see that it was really Opera that time. Yeah.

As browser sniffers got even sniffier and started excluding any browser that said 'Opera' /anywhere/ in the UA string, Opera had to add the Mask As option. When Opera Masks As IE or Firefox, all reference to Opera vanishes from the UA and also from any JS sniffing that's done for Opera-specific items or omissions.