Is the commentary to this masechta attributed to somebody other than Rashi?
Nahum Cohen, Israel
To the best of my knowledge, the Perush alongside the Gemara in Bechoros is indeed Rashi.
I saw in the hakdama to sdai tzofim on bechoros that the avnei nezer wrote that rashi to Bechoros might not be rashi.
Shmuel D Berkovicz
(a) The Avnei Nezer comments in his Teshuvos (YD 53:6) that he is not certain whether Rashi is the actual author of what is printed under his name in Bechoros, since Rashi in Bechoros writes numerous times "mi'Pi ha'Moreh" or "Lo mi'Pi ha'Moreh", a term atypical of Rashi's commentary on the Talmud.
(b) My take on this is that on the one hand, it is certainly true that some Rishonim (e.g. Tosfos Rid) use the word 'ha'Moreh' to refer to Rashi himself.
However, since there are no other hints that point to an alternate authorship of Rashi's commentary on Bechoros, the 'proof' from the words "mi'Pi ha'Moreh" is dubious at best. Rashi does use the term in numerous instances in Shas when citing two explanations for a Gemara ('Lashon Acher' or 'Inyan Acher'), in order to attribute one of the commentaries to his mentor.
In 3 places in Bava Kama, for example, Rashi writes 'mi'Pi ha'Moreh' (23a DH Lechaivo, 37b DH d'Im Ken, 62a DH Mai), sometimes accepting the Perush and sometimes rejecting it. In Menachos, Rashi uses the term in 4 places (42a, 48b, and twice in 51b), in one of which (51b DH Kohanim) he writes that what he is quoting is "Lo mi'Pi ha'Moreh".
Thus, the 5 places where the term appears in Rashi in Bechoros (15b, 28b, 40a, 59a, 59b - two of which are "v'Lo mi'Pi ha'Moreh") are not so exceptional that we may conclude based on this alone that another Rishon authored the commentary of 'Rashi' on Bechoros.
True, I have found other 'strange' expressions in Rashi's commentary on Bechoros: "Lashon Mori b'Shechitas Chulin" (top of 41a), and "Lashon Mori b'Bava Kama" (50b). But even that is not unusual enough to warrant the Avnei Nezer's conclusion - see, for example, Rashi Berachos 39a "b'Perush Bava Kama Shel Rabeinu Yitzchak b'Rebbi Yehudah...".
Best regards and Moadim l'Simcha,