My observed fire resistance of bosses (boss name, resistance observed, number of fire spells cast):
Patchwerk, 117, 405
Grobbulus, 119, 237
Gluth, -, -
Thaddius, 115, 115
Anub'Rekhan, 131, 235
Grand Widow Faerlina, 124, 168
Instructor Razuvious, 126, 183
Gothik the Harvester, 129, 73
Thane Korth'azz, 117, 171
Lady Blameux, 170, 48
Highlord Mograine, 111, 127
Sir Zeliek, 109, 71
Noth the Plaguebringer, 96, 413
Heigan the Unclean, 106, 373
Loatheb, 78, 565
Sapphiron, 102, 758
Kel'Thuzad, 122, 229
Ofc the numbers will still change and on some bosses where I did not do many casts, they are not accurate enough, but you can see the trend. Mb someone share his observations, too, so we can make a sheet to get more precise results?