Just FYI: The cakes between the IE team & the Firefox team have historically been sent to congratulate the recipient for some feat or other (usually a major release), so I can see how the ReactOS team sending the Windows team a cake in conjunction with their own release could be taken wrongly. However, it's a cake. I doubt they're not going to eat it, and as long as it's not a gross cake (make it a delicious and moist cake instead

) those who get to eat it will likely see it as something along the lines of "We mean you no ill-will."
Also I would guess (this is speculation) that the original cake sent by the IE team to the Firefox team was likely meant to indicate two things, 1: 'congratulations, we know you had to work hard to get this far' and 2: 'yes, we see you reintroducing our old rival with improvements and a new name, let's be mature about our rivalry this time around.'
And for those of you out there wondering why I would make the above guesses:
1 In developing Firefox as a standalone browser (separating it from the Mozilla Suite) there were a number of difficulties, both technical and legal, which had to be dealt with appropriately.
2a The Mozilla foundation was Netscape's method of open-sourcing Netscape Navigator (IEs biggest rival for a long time), which Microsoft could have taken as provocation.
2b The Netscape Navigator vs. Microsoft Internet Explorer rivalry included some
very public shenanigans, which I would think Microsoft (and the IE team) would not want to relive.